Patellofemoral joint kinematics and contact pressures in total knee arthroplasty

Medialized Dome and Anatomic Onlay Patella Designs in the Modern Posterior Stabilized Rotating Platform Total Knee Arthroplasty Demonstrate No Clinical or Radiological Differences at One Year

BACKGROUND

A successful outcome after total knee arthroplasty (TKA) includes the restoration of patellofemoral function. Modern patella component designs in TKA include a medialized dome and more recently, an anatomic design. There is a paucity of literature comparing these 2 implants.

METHODS

This prospective nonrandomized study examined 544 consecutive TKAs with patella resurfacing using a posterior-stabilized, rotating platform knee prothesis performed by a single surgeon. A medialized dome patella design was used in the first 323 cases and an anatomic design in the subsequent 221 cases. Patients were assessed preoperatively, at 4 weeks and 1-year post-TKA for Oxford Knee Score (OKS) (total, pain, and kneeling components) and range of movement (ROM). Radiolucent lines (RLLs), patellar tilts and displacements, as well as reoperations were assessed at 1-year post-TKA.

RESULTS

At 1-year post-TKA, both groups demonstrated similar improvements in ROM, OKS, pain, and kneeling scores, and had a similar incidence of fixed-flexion deformity (all P > .05). Radiographically, there was no clinically significant difference in the incidence of RLLs, patellar tilts, and displacements. The prevalence of reoperations (1.8 versus 3.2%, P = .526) was similar between the designs with no patella-related complications.

CONCLUSION

Both medialized dome and anatomic patella designs result in improved ROM and OKS with no patella-related complications. However, our study showed no differences between the designs at 1 year.

Factors that impact the patellofemoral contact stress in the TKA: a review

Abnormal retro patellar stress is believed to contribute to patellofemoral complications after total knee arthroplasty (TKA), but the causal link between TKA and patellofemoral contact stress remains unclear. By reviewing the relevant studies, we found that both TKA implantation and additional patellar resurfacing increase retro patellar pressure. The rotation and size of the femoral component, thickness and position of the patellar component, installation of the tibial component, prosthesis design and soft tissue balance further influence patellofemoral stress. Specific measures can be applied to reduce stress, including the installation of the femoral prosthesis with an appropriate external rotation angle, placing the tibial component at a more posterior position and the patellar button at a more medial position, avoiding over-sized femoral and patellar components, selecting posterior-stabilized design rather than cruciate-retaining design, using gender-specific prosthesis or mobile-bearing TKA system, and releasing the lateral retinaculum or performing partial lateral facetectomy. Despite these measures, the principle of individualization should be followed to optimize the patellofemoral biomechanics.

Higher patellofemoral compressive force does not affect anterior knee pain in anatomical bi-cruciate retaining total knee arthroplasty: In vivo prospective analysis of guided motion prosthesis

BACKGROUND

The purposes of the present study are 1) to measure intraoperative patellofemoral compressive force in patients undergoing anatomical bi-cruciate retaining total knee arthroplasty and to assess the relationship between intraoperative patellofemoral compressive force and patient reported outcome measurements and 2) to compare patellofemoral compressive force and patient reported outcome measurements among patients who underwent anatomical bi-cruciate retaining, cruciate retaining, and bi-cruciate stabilized total knee arthroplasty.

METHODS

Twenty-two patients with varus osteoarthritis of the knee who underwent anatomical bi-cruciate retaining total knee arthroplasty, 20 patients who underwent cruciate retaining total knee arthroplasty, and 24 patients who underwent bi-cruciate stabilized total knee arthroplasty were assessed. Patient reported outcome measurements were evaluated at 1.5 years after surgery.

FINDINGS

Intraoperative patellofemoral compressive force was significantly lower with anatomical bi-cruciate retaining total knee arthroplasty than with cruciate retaining total knee arthroplasty at 60° to 140° of flexion and nearly equivalent to intraoperative patellofemoral compressive force with bi-cruciate stabilized total knee arthroplasty at all knee flexion angles examined. With anatomical bi-cruciate retaining total knee arthroplasty, there were no significant correlations between intraoperative patellofemoral compressive force and almost all patient reported outcome measurements except for 2011 Knee Society Score expectations, which was positively correlated with patellofemoral compressive force at 10° of flexion, and Patella score quadriceps strength, which was negatively correlated with patellofemoral compressive force at 60° of flexion.

INTERPRETATION

There were no significant correlations between intraoperative patellofemoral compressive force and anterior knee pain after anatomical bi-cruciate retaining total knee arthroplasty. Evidence level: 3.

The influence of compressive forces across the patellofemoral joint on patient-reported outcome after bi-cruciate stabilized total knee arthroplasty

AIMS

Patellofemoral problems are a common complication of total knee arthroplasty. A high compressive force across the patellofemoral joint may affect patient-reported outcome. However, the relationship between patient-reported outcome and the intraoperative patellofemoral contact force has not been investigated. The purpose of this study was to determine whether or not a high intraoperative patellofemoral compressive force affects patient-reported outcome.

PATIENTS AND METHODS

This prospective study included 42 patients (42 knees) with varus-type osteoarthritis who underwent a bi-cruciate stabilized total knee arthroplasty and in whom the planned alignment was confirmed on 3D CT. Of the 42 patients, 36 were women and six were men. Their mean age was 72.3 years (61 to 87) and their mean body mass index (BMI) was 24.4 kg/m ² (18.2 to 34.3). After implantation of the femoral and tibial components, the compressive force across the patellofemoral joint was measured at 10°, 30°, 60°, 90°, 120°, and 140° of flexion using a load cell (Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan) manufactured in the same shape as the patellar implant. Multiple regression analyses were conducted to investigate the relationship between intraoperative patellofemoral compressive force and patient-reported outcome two years after implantation.

RESULTS

No patient had anterior knee pain after total knee arthroplasty. The compressive force across the patellofemoral joint at 140°of flexion was negatively correlated with patient satisfaction (R ² = 0.458; β = -0.706; p = 0. 041) and Forgotten Joint Score-12 (FJS-12; R ² = .378; β = -0.636; p = 0. 036). The compressive force across the patellofemoral joint at 60° of flexion was negatively correlated with the patella score (R ² = 0.417; β = -0.688; p = 0. 046).

CONCLUSION

Patient satisfaction, FJS-12, and patella score were affected by the patellofemoral compressive force at 60° and 140° of flexion. Reduction of the patellofemoral compressive forces at 60° and 140° of flexion angle during total knee arthroplasty may improve patient-reported outcome, but has no effect on anterior knee pain.

In vivo comparison of medialized dome and anatomic patellofemoral geometries using subject-specific computational modeling

Successful outcome following total knee arthroplasty (TKA) with patella resurfacing is partly determined by the restoration of patellofemoral (PF) function and recovery of the quadriceps mechanism. The current study compared two patellar TKA geometries (medialized dome and anatomic) to determine their impact on PF mechanics and quadriceps function. In-vivo, subject-specific patellar mechanics were evaluated using a sequential experimental and modeling approach. First, stereo radiography, marker-based motion capture, and force plate data were collected for TKA patients (10 dome, 10 anatomic) performing a knee extension and lunge. Second, subject-specific, whole-body, musculoskeletal models, including 6 degrees-of-freedom (DOF) knee joint kinematics, were created for each subject and activity to predict quadriceps forces. Last, finite element models of each subject and activity were created to predict PF kinematics, patellar loading, moment arm, and patellar tendon angle. Differences in mechanics between dome and anatomic patients were highlighted during load-bearing (lunge) activity. Anatomic subjects demonstrated greater PF flexion angles (avg. 11 ± 3°) compared to dome subjects during lunge. Similar to the natural knee, contact locations on the patella migrated inferior to superior as the knee flexed in anatomic subjects, but remained relatively superior in dome subjects. Differences in kinematics and contact location likely contributed to altered mechanics with anatomic subjects presenting greater load transfer from the quadriceps to the patellar tendon in deep flexion (>75°), and dome subjects demonstrating larger contact forces during lunge. Although there was substantial patient variability, evaluations of PF mechanics suggested improved quadriceps function and more natural kinematics in the anatomic design. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1910-1918, 2018.

Medial stabilized and posterior stabilized TKA affect patellofemoral kinematics and retropatellar pressure distribution differently

PURPOSE

Patellofemoral kinematics and retropatellar pressure distribution change after total knee arthroplasty (TKA). It was hypothesized that different TKA designs will show altered retropatellar pressure distribution patterns and different patellofemoral kinematics according to their design characteristics.

METHODS

Twelve fresh-frozen knee specimens were tested dynamically in a knee rig. Each specimen was measured native, after TKA with a posterior stabilized design (PS) and after TKA with a medial stabilized design (MS). Retropatellar pressure distribution was measured using a pressure sensitive foil which was subdivided into three areas (lateral and medial facet and patellar ridge). Patellofemoral kinematics were measured by an ultrasonic-based three-dimensional motion system (Zebris CMS20, Isny Germany).

RESULTS

Significant changes in patellofemoral kinematics and retropatellar pressure distribution were found in both TKA types when compared to the native situation. Mean retropatellar contact areas were significantly smaller after TKA (native: 241.1 ± 75.6 mm², MS: 197.7 ± 74.5 mm², PS: 181.2 ± 56.7 mm², native vs. MS p < 0.001; native vs. PS p < 0.001). The mean peak pressures were significantly higher after TKA. The increased peak pressures were however seen in different areas: medial and lateral facet in the PS-design (p < 0.001), ridge in the MS design (p < 0.001). Different patellofemoral kinematics were found in both TKA designs when compared to the native knee during flexion and extension with a more medial patella tracking.

CONCLUSION

Patellofemoral kinematics and retropatellar pressure change after TKA in different manner depending on the type of TKA used. Surgeons should be aware of influencing the risks of patellofermoral complications by the choice of the prosthesis design.

Morphological classification of the femoral trochlear groove based on a quantitative measurement of computed tomographic models

PURPOSE

This study aimed to investigate the natural morphology of the femoral trochlear groove based on quantitative measurement.

METHODS

Computed tomographic femur models of 50 male and 50 female healthy Chinese adults (30-60 years) were analysed using three-dimensional software. Coaxial cutting planes (15° increments) rotating about the trochlear groove axis from the proximal to distal point were created, followed by the deepest point of the trochlear groove marked at each cross section. The shape, position, and orientation of the trochlear groove were analysed.

RESULTS

The trochlear groove was located laterally relative to the mechanical axis and consisted of the laterally oriented proximal part and medially oriented distal part. Based on the turning points located on different cross sections, the trochlear groove was classified into four types: types 45°, 60°, 75°, and 90°. The mediolateral position relative to the mechanical axis was types 45°, 60°, 75°, and 90°, from the lateral to medial side, while the distal parts of them extended along the same path. The orientation of the trochlear groove was relatively consistent and smooth, which oriented at approximately 1° medially between two adjacent segments, except at approximately 10° medially at the turning point.

CONCLUSION

The trochlear groove tracking varies greatly amongst a population that is mainly categorized into four types. This study may be helpful for better understanding of the natural trochlear groove anatomy, prosthetic design modification, and provide the reference value for studying patellofemoral diseases such as patellar maltracking and trochlear dysplasia.

LEVEL OF EVIDENCE

Prospective study, Level II.

Differences between native and prosthetic knees in terms of cross-sectional morphology of the femoral trochlea: a study based on three-dimensional models and virtual total knee arthroplasty

Background

The cross-sectional morphology of the prosthetic knee is crucial to understanding patellar motion and quadriceps strength after total knee arthroplasty. However, few comparative evaluations of the cross-sectional morphology of the femoral trochlea have been performed in the native knee and currently available femoral implants, and the relationship between the trochlear anatomy of prosthetic components and post-operative patellofemoral complications remains unclear. We aimed to investigate the differences in cross-sectional morphology of the femoral trochlea between native knees and prosthetic femoral components.

Methods

Virtual total knee arthroplasty was performed, whereby four different femoral components (medial-pivot, Triathlon, NRG and NexGen) were virtually superimposed onto three-dimensional models of 42 healthy femurs. The following morphological parameters were measured in three cross-sections (0, 45 and 90°) of the femoral trochlea: sulcus height, lateral tilt angle, medial tilt angle and sulcus angle. Only statistically significant differences are described further (p < 0.05).

Results

In the 0° cross-section, sulcus height was smaller in the native knee than in the Triathlon, NRG and NexGen components; all prosthetic components had smaller lateral tilt angles and larger medial tilt angles. In the 45° cross-section, sulcus height was larger in the native knee than in the medial-pivot, Triathlon and NexGen components; both lateral and medial tilt angles were smaller in the prosthetic components. In the 90° cross-section, sulcus height was smaller in the native knee than in the medial-pivot component; all prosthetic components had a larger lateral tilt angle and smaller medial tilt angle. In all cross-sections, the sulcus angle was smaller in the native knee.

Conclusions

The discrepancy between native and prosthetic trochlear geometries suggests altered knee mechanics after total knee arthroplasty, but further cadaveric, computational or fluoroscopic investigations are necessary to clarify the implications of this observation. Our findings can be used to optimize biomechanical guidelines for total knee arthroplasty (patellar resurfacing or non-resurfacing) in Chinese individuals so as to decrease the risk of patellar lateral dislocation, to maintain stability and to optimize extensor kinematics.

In Vivo Three-Dimensional Patellar Mechanics: Normal Knees Compared with Domed and Anatomic Patellar Components

BACKGROUND

Patellofemoral complications are a major cause of revision surgery following total knee arthroplasty (TKA). High forces occurring at the patellofemoral articulation coupled with a small patellofemoral contact area pose substantial design challenges. In this study, the three-dimensional (3D) in vivo mechanics of domed and anatomically shaped patellar components were compared with those of native patellae.

METHODS

Ten normal knees, 10 treated with an LCS-PS (low contact stress-posterior stabilized) TKA (anatomically shaped patellar component), and 10 treated with a PFC Sigma RP-PS (press-fit condylar Sigma rotating platform-posterior stabilized) TKA (domed patellar component) were analyzed under fluoroscopic surveillance while the patient performed a weight-bearing deep knee bend from full knee extension to maximum knee flexion. Relevant bone geometries were segmented out from computed tomography (CT) scans, and computer-assisted-design (CAD) models of the implanted components were obtained from the manufacturer. Three-dimensional patellofemoral kinematics were obtained using a 3D-to-2D registration process. Contact mechanics were calculated using a distance map between the articulating patellar and femoral surfaces.

RESULTS

Both patellar component designs exhibited good rotational kinematics and tracked well within the femoral trochlea when compared with the normal patella. The contact areas in the TKA groups peaked at 60° of knee flexion (mean and standard deviation, 201 ± 63.4 mm for the LCS-PS group and 218 ± 95.4 mm for the Sigma RP-PS group), and the areas were substantially smaller than those previously reported for the normal patella. Contact points in the TKA groups stayed close to the center of the patellar components.

CONCLUSIONS

Both designs performed satisfactorily, although patellofemoral contact areas were reduced in comparison with those in the native patella.

LEVEL OF EVIDENCE

Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Influence of Total Knee Arthroplasty on Patellar Kinematics and Patellofemoral Pressure

BACKGROUND

Patellofemoral complications are one of the main problems after total knee arthroplasty (TKA). The design of the TKA component may affect the patellar biomechanics, which may be associated with this postoperative complication. The purpose of this study was to assess the influence of TKA and prosthesis designs on the patellar kinematics and patellofemoral pressure.

METHODS

Using fresh-frozen cadavers, we measured the patellofemoral pressure, patella offset, and patella tilt in the following 4 conditions: normal knee (patella replacement only), cruciate-retaining TKA, condylar-stabilizing TKA, and posterior-stabilized TKA.

RESULTS

The patellofemoral pressure increased significantly after the cruciate-retaining TKA and condylar-stabilizing TKA compared with the normal knee. The patella offset in the normal knee decreased with increasing knee flexion angles, while the patella offset in the TKA knees did not change significantly through the full range of motion. The amount of lateral patella tilt in the normal knee was significantly larger than the TKA knees in the full range of motion.

CONCLUSION

Although the femoral components are designed to reproduce an anatomical patellar tracking, the physiological patellar kinematics were not observed. Relatively high patellofemoral pressure and kinematic change after TKA may be associated with postoperative complications such as the anterior knee pain.

In vitro method for assessing the biomechanics of the patellofemoral joint following total knee arthroplasty

The patellofemoral joint is a common site of pain and failure following total knee arthroplasty. A contributory factor may be adverse patellofemoral biomechanics. Cadaveric investigations are commonly used to assess the biomechanics of the joint, but are associated with high inter-specimen variability and often cannot be carried out at physiological levels of loading. This study aimed to evaluate the suitability of a novel knee simulator for investigating patellofemoral joint biomechanics. This simulator specifically facilitated the extended assessment of patellofemoral joint biomechanics under physiological levels of loading. The simulator allowed the knee to move in 6 degrees of freedom under quadriceps actuation and included a simulation of the action of the hamstrings. Prostheses were implanted on synthetic bones and key soft tissues were modelled with a synthetic analogue. In order to evaluate the physiological relevance and repeatability of the simulator, measurements were made of the quadriceps force and the force, contact area and pressure within the patellofemoral joint using load cells, pressure-sensitive film, and a flexible pressure sensor. The results were in agreement with those previously reported in the literature, confirming that the simulator is able to provide a realistic physiological loading situation. Under physiological loading, average standard deviations of force and area measurements were substantially lower and comparable to those reported in previous cadaveric studies, respectively. The simulator replicates the physiological environment and has been demonstrated to allow the initial investigation of factors affecting patellofemoral biomechanics following total knee arthroplasty.

Correlation between knee kinematics and patellofemoral contact pressure in total knee arthroplasty

The aim of this study is to evaluate the relationship between patellofemoral contact stress and intraoperative knee kinematic patterns after mobile bearing total knee arthroplasty (TKA). Medial osteoarthritic knees of forty-six posterior-stabilized total knee prostheses were evaluated using a computed tomography-guided navigation system. Subjects were divided into two groups based on intraoperative knee kinematic patterns: the medial pivot group (n=19) and the non-medial pivot group (n=27). Mean intraoperative patello-femoral contact stress was significantly lower in the medial pivot group than in the non-medial pivot group (1.7MPa vs. 3.2MPa, P<0.05). An intraoperative medial pivot pattern results in reduced patello-femoral contact stress.

The biomechanical effect of increased valgus on total knee arthroplasty: a cadaveric study

The effects of valgus load on cadaveric knees following total knee arthroplasty (TKA) were investigated using a custom testing system. TKAs were performed on 8 cadaveric knees and tested at 0°, 30°, and 60° knee flexion in both neutral and 5° valgus. Fuji pressure sensitive film was used to quantify contact areas and pressures and MCL strain was determined using a Microscribe digitizing system. Lateral tibiofemoral pressures increased (P < 0.05) at all knee flexion angles with valgus loading. Patellofemoral contact characteristics did not change significantly (P > 0.05). Significant increases in strain were observed along the anterior and posterior border of the MCL at all knee flexion angles. These findings suggest that valgus loading increases TKA joint contact pressures and MCL strain with increasing knee flexion which may increase implant instability.

Decreased ratios of lateral to medial patellofemoral forces and pressures after lateral retinacular release and gender knees in total knee arthroplasty

PURPOSE

To demonstrate that lateral to medial patellofemoral force and pressure ratios could be a surrogate marker of retinacular tension and patellar tracking.

METHODS

The patellofemoral forces of six knees from three fresh-frozen half-body female cadavers were evaluated with a capacitive sensor under simulated operative conditions in six staged clinical scenarios: native knees, knee arthroplasty without patellar resurfacing, resurfaced knee and patella, resurfaced knee and patella with lateral release, gender-specific knee arthroplasty with patella resurfacing, and gender-specific knee arthroplasty with lateral release. Maximum force and peak pressure were simultaneously recorded during three to four ranges of motion. Average values were compared between lateral and medial patellofemoral compartments as an objective measure of patellar tracking for the different settings.

RESULTS

Significant differences in lateral and medial force and pressure differentials were seen in most scenarios despite clinically normal patellar tracking. Lateral to medial ratios of maximum force and peak pressure significantly increased after TKA (2.9, 2.1) and after patella resurfacing (2.8, 2.6) compared to the native knee (1.6, 1.8). Addition of a lateral release in resurfaced knees decreased the ratio of lateral to medial patellofemoral forces and pressures as did gender knee arthroplasty (1.5 and 1.1, 2 and 1.3, respectively). Pressure and force values most closely resembled the native knee in the resurfaced knee/resurfaced patella with lateral release and in the gender knee arthroplasty scenarios.

CONCLUSIONS

Use of lateral to medial patellofemoral force ratios as a surrogate objective marker for patellar tracking was validated in this study by decreasing ratios observed after lateral release in TKA and with gender-specific implants.

The influence of patellar resurfacing on patellar kinetics and retropatellar contact characteristics

BACKGROUND

Femoropatellar complications are one of the most common problems after total knee arthroplasty (TKA). However, the question of whether to resurface the patella remains controversial. Therefore, we evaluated the kinetics and the retropatellar contact characteristics of patella resurfacing with fixed and gliding surfaces.

METHODS

Eight Thiel-embalmed cadaver knees were tested--first intact, then after TKA without patellar resurfacing, and finally with additional patellar resurfacing--while flexing the knee from 0° to 100°. We tested a fixed as well as a gliding patella surface. During the examination, quadriceps and hamstring forces were applied. The retropatellar pressure was determined with a special patella sensor, and the patellar kinetics were measured using an optical three-dimensional motion analysis system.

RESULTS

Resurfacing the patella caused a significant increase in retropatellar pressure and a significant decrease in retropatellar contact area. Using a fixed patella, the retropatellar pressure nearly quadrupled in higher flexion compared to the native patella. Furthermore, the lateral movement of the patella increased after TKA, especially after additional patellar resurfacing.

CONCLUSIONS

Resurfacing the patella routinely is not advised. When osteoarthritis of the patella is found, the gliding patella should be preferred.

The influence of patella height on soft tissue balance in cruciate-retaining and posterior-stabilised total knee arthroplasty

PURPOSE

Although the patella reduced or everted position has recently been recognised as an important factor influencing soft tissue balance during assessment in total knee arthroplasty (TKA), the influence of patella height on soft tissue balance has not been well addressed. Therefore, the relationship between soft tissue balance and patella height was investigated and differences between cruciate-retaining (CR) and posterior-stabilised (PS) TKA were compared.

METHODS

Forty consecutive patients blinded to the type of implant received, were randomised prospectively. Using lateral radiographs, pre-operative patella height was measured. Using an offset-type tensor designed to measure the soft tissue balance with a reduced patellofemoral (PF) joint and femoral component in place, soft tissue balance was intra-operatively assessed in CR TKA (n = 20) and PS TKA (n = 20) in osteoarthritic patients. The joint component gap and varus ligament balance at zero, ten, 45, 90 and 135° of knee flexion with the patella reduced were measured.

RESULTS

In PS TKA, the joint component gap positively correlated with patella height at 90 and 135° of knee flexion. However, there was no correlation between joint component gap and patella height at other flexion angles in PS TKA and any flexion angle in CR TKA. Varus ligament balance showed no significant correlation with patella height in either CR or PS TKA.

CONCLUSION

Analysis of soft tissue balance and patella height only showed a positive correlation in joint component gap at a high flexion angle (90 and 135°) in PS TKA but not in other parameters examined. Pre-operative measurement of patella height may be an important factor for predicting an intra-operative flexion gap in PS TKA.

Tibiofemoral and patellofemoral joint 3D-kinematics in patients with posterior cruciate ligament deficiency compared to healthy volunteers

BACKGROUND

The posterior cruciate ligament (PCL) plays an important role in maintaining physiological kinematics and function of the knee joint. To date mainly in-vitro models or combined magnetic resonance and fluoroscopic systems have been used for quantifying the importance of the PCL. We hypothesized, that both tibiofemoral and patellofemoral kinematic patterns are changed in PCL-deficient knees, which is increased by isometric muscle flexion. Therefore the aim of this study was to simultaneously investigate tibiofemoral and patellofemoral 3D kinematics in patients suffering from PCL deficiency during different knee flexion angles and under neuromuscular activation.

METHODS

We enrolled 12 patients with isolated PCL-insufficiency as well as 20 healthy volunteers. Sagittal MR-images of the knee joint were acquired in different positions of the knee joint (0°, 30°, 90° flexion, with and without flexing isometric muscle activity) on a 0.2 Tesla open MR-scanner. After segmentation of the patella, femur and tibia local coordinate systems were established to define the spatial position of these structures in relation to each other.

RESULTS

At full extension and 30° flexion no significant difference was observed in PCL-deficient knee joints neither for tibiofemoral nor for patellofemoral kinematics. At 90° flexion the femur of PCL-deficient patients was positioned significantly more anteriorly in relation to the tibia and both, the patellar tilt and the patellar shift to the lateral side, significantly increased compared to healthy knee joints. While no significant effect of isometric flexing muscle activity was observed in healthy individuals, in PCL-deficient knee joints an increased paradoxical anterior translation of the femur was observed at 90° flexion compared to the status of muscle relaxation.

CONCLUSIONS

Significant changes in tibiofemoral and patellofemoral joint kinematics occur in patients with isolated PCL-insufficiency above 30 degrees of flexion compared to healthy volunteers. Since this could be one reasonable mechanism in the development of osteoarthritis (OA) our results might help to understand the long-term development of tibiofemoral and/or patellofemoral OA in PCL-insufficient knee joints.

The influence of patella height on intra-operative soft tissue balance in posterior-stabilized total knee arthroplasty

PURPOSE

Although the patella reduced or everted position has recently been recognized as an important factor in influencing soft tissue balance during the assessment in total knee arthroplasty (TKA), the influence of patella height on soft tissue balance has not been well addressed. We therefore investigated the effect of patella height by comparing soft tissue balance between high [Insall-Salvati index (ISI) > 1] and low patella patients (ISI ≦ 1).

METHODS

Using a tensor designed to facilitate soft tissue balance measurements with a reduced patellofemoral joint and femoral component in place, we intra-operatively assessed the joint component gap and ligament balance of posterior-stabilized (PS) TKAs in 30 osteoarthritic patients performed at 0, 10, 45, 90, and 135° of flexion, with the patella reduced.

RESULTS

When comparing the two groups, the component gaps of the higher patella group showed a larger trend than those of the lower patella group, with significant differences at 90 and 135° of knee flexion. Moreover, the joint component gap positively correlated with ISI at 90 and 135° of knee flexion.

CONCLUSION

Patella higher group showed significant larger component gaps than patella lower group in high flexion angles (90 and 135°). Pre-operative measurement of patellar height can help predict intra-operative soft tissue balance in PS TKA.

LEVEL OF EVIDENCE

III.

Biomechanical effects of joint line elevation in total knee arthroplasty

BACKGROUND

Inadequate restoration of the knee joint line after total knee arthroplasty may lead to a poor clinical outcome. The purpose of this study was to quantitatively assess the effects of joint line elevation following total knee arthroplasty with increased joint volume on patellofemoral contact kinematics.

METHODS

Six cadaveric specimens were tested. Patellofemoral contact area, contact pressure, and kinematics were measured following total knee arthroplasty with an anatomic joint line and after 4 and 8mm of joint line elevation, at knee flexion angles of 0°, 30°, 60°, 90° and 120°. Repeated measures analysis of variance with a Tukey post hoc test with a significance level of 0.05 was used for statistical analyses.

FINDINGS

There was a decrease in contact area with joint line elevation at flexion angles of 60°, 90° and 120° (P=0.009-0.04). There was a significant increase in contact pressure only at 30° of knee flexion with 8mm of joint line elevation (P=0.004). Three of the six specimens showed inferior edge loading of the patella component following 8mm of joint line elevation at 120° of knee flexion. The sagittal plane patellofemoral angle increased significantly with joint line elevation except for 0° knee flexion (P=0.0002-0.02).

INTERPRETATION

Knee joint line elevation with increased knee volume significantly affects patellofemoral contact area and kinematics and produced inferior edge loading/impingement between the patella and tibial components, this may result in loss of knee range of motion, postoperative pain, and premature component wear.

Comparison of patellar retention versus resurfacing in LCS mobile-bearing total knee arthroplasty

PURPOSE

The aim of this retrospective study was to compare clinical outcomes of total knee arthroplasty (TKA) with and without patellar resurfacing using the Low Contact Stress (LCS) mobile-bearing prosthesis in 275 osteoarthritic knees (199 patients) after a minimum of 7 years of follow-up.

METHODS

Patients were divided into a patellar retention group (132 knees) and a resurfacing group (143 knees), with median follow-up durations of 7.8 years (range, 7-8.5 years) and 8.5 years (range, 7-10.6 years), respectively. The demographics of the two groups were otherwise matched. The patelloplasty was performed for patellar retention. Patients were evaluated by a blinded, independent observer using Feller's patellar score, the Knee Society score, patient satisfaction, patellar tilt, and lateral displacement.

RESULTS

Patellar resurfacing was not superior to retention with respect to any of the measured variables. Eight knees (6.1%) without and 6 (4.2%) with patellar resurfacing had anterior knee pain related to the patellofemoral joint (n.s.). The reoperation rate related to the patellofemoral joint was 0.8% (n = 1) in the retention group compared with 2.8% (n = 4) in the resurfacing group (n.s.). In the 35 patients who underwent bilateral TKA with patellar resurfacing on only one side, there were no significant differences between the two sides in subjective preference, clinical scores, or functional ability.

CONCLUSIONS

The clinical and radiographic outcomes of TKA with the LCS mobile-bearing prosthesis showed no significant difference between the two groups after a minimum of 7 years of follow-up. The findings in this study suggest that patellar retention with a patelloplasty may be viable as a routine procedure, even in knees with advanced patellofemoral arthritis, if soft tissue balancing and a patella-friendly prosthetic design are properly used.

LEVEL OF EVIDENCE

Therapeutic, retrospective, comparative study, Level III.

Patellofemoral joint contact forces during activities with high knee flexion

The patellofemoral (PF) joint plays an essential role in knee function, but little is known about the in vivo loading conditions at the joint. We hypothesized that the forces at the PF joint exceed the tibiofemoral (TF) forces during activities with high knee flexion. Motion analysis was performed in two patients with telemetric knee implants during walking, stair climbing, sit-to-stand, and squat. TF and PF forces were calculated using a musculoskeletal model, which was validated against the simultaneously measured in vivo TF forces, with mean errors of 10% and 21% for the two subjects. The in vivo peak TF forces of 2.9-3.4 bodyweight (BW) varied little across activities, while the peak PF forces showed significant variability, ranging from less than 1 BW during walking to more than 3 BW during high flexion activities, exceeding the TF forces. Together with previous in vivo measurements at the hip and knee, the PF forces determined here provide evidence that peak forces across these joints reach values of around 3 BW during high flexion activities, also suggesting that the in vivo loading conditions at the knee can only be fully understood if the forces at the TF and the PF joints are considered together.

Influence of total knee arthroplasty on patellar kinematics and contact characteristics

PURPOSE

Femoro-patellar complications are one of the most common problems after total knee arthroplasty (TKA). TKA components that reduce patellar loads and preserve physiological patellar kinematics should reduce these problems. Therefore, we evaluated the patellar kinematics and the retro-patellar contact characteristics in both the intact knee and in the TKA-knee.

METHODS

Eight Thiel-embalmed cadaver knees were tested first intact and then after TKA using rotating as well as gliding inlay and with additional patellar resurfacing while flexing the knee from 0° to 100°. During the examination quadriceps and hamstring forces were applied.

RESULTS

TKA with additional patellar resurfacing led to an increased retro-patellar pressure, a decreased contact area and an increased lateral movement. Although patellar kinematics could not be changed by using a gliding inlay compared to a rotating inlay, the gliding inlay improved retro-patellar contact characteristics by reducing the pressure and increasing the contact area, especially in higher flexion.

CONCLUSIONS

The increased retro-patellar pressure together with the increased lateral movement of the patella after TKA may be one important cause for anterior knee pain appearing after TKA. In view of the improved retro-patellar contact characteristics using a gliding inlay this inlay should be preferred, providing that the posterior cruciate ligament is intact.

Comparison of in vivo patellofemoral kinematics for subjects having high-flexion total knee arthroplasty implant with patients having normal knees

This study compares the in vivo patellar kinematics of high-flexion posterior cruciate ligament-retaining and posterior-stabilized total knee arthroplasty (TKA) implants with that of the healthy knee. Twenty-seven subjects performing weight-bearing deep knee bends were analyzed under fluoroscopic surveillance from full extension to maximum flexion. The patellofemoral contact positions and patellar flexion were similar for both TKAs. At low flexion, the patellofemoral contact was significantly more distal on the healthy patella than on the TKA patella, but in deeper flexion, there was no difference among the 3 groups. The tibiopatellar angle was similar for all 3 groups, except at deep flexion where the healthy patella rotated significantly more than the implanted ones. Patellofemoral separation was observed in some TKA knees, whereas it was absent in the healthy knees.

Magnetic resonance imaging of in vivo patellofemoral kinematics after total knee arthroplasty

Simulated partial weight bearing during magnetic resonance imaging of the knee was used to measure patellar tilt, medial-lateral patellar shift, and patellofemoral contact area in three groups of subjects; patients with posterior cruciate retaining (PCR) TKA, patients with bicruciate substituting (BCS) TKA, and healthy controls. Contact stress was also calculated based on the contact area and body weight-based estimates of contact force. Contact stress was significantly (p<0.05) higher in PCR knees (2.5+/-3.0 MPa) than in BCS knees (0.2+/-0.1 MPa) when knees were fully extended, but this difference was not significant (3.7+/-3.5 MPa for PCR knees vs. 1.4+/-1.9 MPa for BCS knees; p>0.05) in early flexion. The results also indicate that patellar tilt (normal=2.4 degrees +/-4.8 degrees, BCS=5.5 degrees +/-5.5 degrees, PCR=-3.0 degrees +/-6.9 degrees change in lateral tilt when moving from full extension to early flexion) and contact area (full extension: normal=267+/-111 mm(2), BCS=344+/-201 mm(2), PCR=83+/-80 mm(2); early flexion: normal=723+/-306 mm(2), BCS=417+/-290 mm(2), PCR=246+/-108 mm(2)) in BCS TKA mimic those in the normal knees more closely than PCR knees do. These results suggest that the patellar component in BCS TKA may be expected to experience less wear than the patellar component in PCR TKA over time.

Effect of calibration method on Tekscan sensor accuracy

Tekscan pressure sensors are used in biomechanics research to measure joint contact loads. While the overall accuracy of these sensors has been reported previously, the effects of different calibration algorithms on sensor accuracy have not been compared. The objectives of this validation study were to determine the most appropriate calibration method supplied in the Tekscan program software and to compare its accuracy to the accuracy obtained with two user-defined calibration protocols. We evaluated the calibration accuracies for test loads within the low range, high range, and full range of the sensor. Our experimental setup used materials representing those found in standard prosthetic joints, i.e., metal against plastic. The Tekscan power calibration was the most accurate of the algorithms provided with the system software, with an overall rms error of 2.7% of the tested sensor range, whereas the linear calibrations resulted in an overall rms error of up to 24% of the tested range. The user-defined ten-point cubic calibration was almost five times more accurate, on average, than the power calibration over the full range, with an overall rms error of 0.6% of the tested range. The user-defined three-point quadratic calibration was almost twice as accurate as the Tekscan power calibration, but was sensitive to the calibration loads used. We recommend that investigators design their own calibration curves not only to improve accuracy but also to understand the range(s) of highest error and to choose the optimal points within the expected sensing range for calibration. Since output and sensor nonlinearity depend on the experimental protocol (sensor type, interface shape and materials, sensor range in use, loading method, etc.), sensor behavior should be investigated for each different application.

Effect of an UHMWPE patellar component on stress fields in the patella: a finite element analysis

An increased stress in the patella due to the implantation of a patellar button may also be another potential source of pain in total knee arthroplasty patients. This study assessed the location inside the patella having largest stress change after implantation of an ultra high molecular polyethylene patella button. Finite elements models of the patellae before and after implantation of patellar button were created. Experimentally determined spring constants of muscles and ligaments, and patellofemoral contacting loads were applied to the models at 30 degrees , 60 degrees , and 90 degrees of knee flexion. The Von Mises stress of the intact patella decreased with increased knee flexion, while that of implanted patella increased. Also, the stress range in the implanted patella was 3-9 times higher than in the intact one. The highly stressed region of the intact patella moved proximally with higher knee flexion angles, while that of the implanted model stayed near the central anterior patella. At 90 degrees of knee flexion, the stress in the anterodistal patella increased considerably after implantation of a patella button so that the anterodistal patella may be susceptible to be painful source after the total knee replacement.

Determinants of patellar tracking in total knee arthroplasty

BACKGROUND

Optimizing patellar tracking in total knee arthroplasty is a surgical priority. Despite this, a comparison of the effects of different component placements on patellar tracking is not available; the biomechanical impact of the patellar resection angle has not been studied; and the similarity between intraoperative and postoperative effects, fundamental to improving patellar tracking, is unknown. Our objective was to compare the impact of the major controllable femoral, tibial and patellar component positions on patellar kinematics during both passive and loaded flexion.

METHODS

We tested eight cadaveric knee specimens in two rigs, simulating intraoperative and weightbearing flexion. Optoelectronic marker arrays were attached to the femur, tibia and patella to record kinematics throughout the range of motion. We modified posterior-stabilized fixed-bearing knee components to allow for five types of variations in component placement in addition to the neutral position: femoral component rotation, tibial component rotation, patellar resection angle, patellar component medialization and additional patellar thickness, for a total of 11 individual variations.

FINDINGS

The major determinants of patellar tilt and shift were patellar component medialization, patellar resection angle and femoral component rotation. The relative order of these variables depended on the structure (bone or component), kinematic parameter (tilt or shift) and flexion angle (early or late flexion). Effects of component changes were consistent between the intraoperative and weightbearing rigs.

INTERPRETATION

To improve patellar tracking, and thereby the clinical outcome, surgeons should focus on patellar component medialization, patellar resection angle and femoral component rotation. These have been linked with anterior knee pain as well. Neither tibial component rotation nor patellar thickness should be adjusted to improve patellar tracking.

Total knee replacement with and without patellar resurfacing: a prospective, randomised trial using the profix total knee system

We have examined the differences in clinical outcome of total knee replacement (TKR) with and without patellar resurfacing in a prospective, randomised study of 181 osteoarthritic knees in 142 patients using the Profix total knee system which has a femoral component with features considered to be anatomical and a domed patellar implant. The procedures were carried out between February 1998 and November 2002. A total of 159 TKRs in 142 patients were available for review at a mean of four years (3 to 7). The patients and the clinical evaluator were blinded in this prospective study. Evaluation was undertaken annually by an independent observer using the knee pain scale and the Knee Society clinical rating system. Specific evaluation of anterior knee pain, stair-climbing and rising from a seated to a standing position was also undertaken. No benefit was shown of TKR with patellar resurfacing over that without resurfacing with respect to any of the measured outcomes. In 22 of 73 knees (30.1%) with and 18 of 86 knees (20.9%) without patellar resurfacing there was some degree of anterior knee pain (p = 0.183). No revisions related to the patellofemoral joint were performed in either group. Only one TKR in each group underwent a re-operation related to the patellofemoral joint. A significant association between knee flexion contracture and anterior knee pain was observed in those knees with patellar resurfacing (p = 0.006).

Joint gap changes with patellar tendon strain and patellar position during TKA

Balancing of the joint gap in extension and flexion is a prerequisite for success of a total knee arthroplasty. The joint gap is influenced by patellar position. We therefore hypothesized the state of the knee extensor mechanism (including the patellar tendon) would influence the joint gap. In 20 knees undergoing posterior-stabilized type total knee arthroplasties, we measured the joint gap and the patellar tendon strain from 0 degrees to 135 degrees flexion with the femoral component in position. When the patella was reduced, the joint gap was decreased at 90 degrees and 135 degrees (by 1.9 mm and 5.5 mm, respectively) compared with the gap with the patella everted. The patellar tendon strain increased with knee flexion. Patellar tendon strain at 90 degrees flexion correlated with the joint gap difference with the patella in everted and reduced positions. This suggests that in addition to the collateral ligaments, the knee extensor mechanism may have an influence on the joint gap. Therefore, accounting for extensor mechanism tightness may be important in achieving the optimal joint gap balance during total knee arthroplasty.

LEVEL OF EVIDENCE

Level IV, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Biomechanical effects of kneeling after total knee arthroplasty

BACKGROUND

Kneeling following total knee arthroplasty can be a difficult task, impairing the activities of patients to varying degrees. Little is known about the biomechanical effects of kneeling following total knee replacement. The objective of this study was to quantify the effects of kneeling on patellofemoral joint contact areas and pressures, knee joint reaction force, and patellar kinematics.

METHODS

Total knee arthroplasties were performed on eight fresh-frozen cadaveric knees, and they were tested with use of a custom knee jig, which permits the simulation of physiologic quadriceps loading as well as the application of an anterior force to simulate kneeling. The knees were tested at flexion angles of 90 degrees , 105 degrees , 120 degrees , and 135 degrees with no anterior force (mimicking a squatting position) and with an anterior force application simulating double-stance kneeling and single-stance kneeling. Patellofemoral joint contact areas and pressures were measured with pressure-sensitive film, and the knee joint reaction force was measured with use of a six-degree-of-freedom load cell. Patellar kinematics were assessed with use of digital photographs tracking fixed markers on the patella.

RESULTS

Compared with the condition without kneeling, both single-stance and double-stance kneeling demonstrated significant increases in patellofemoral contact area (p < 0.05) and pressure at all flexion angles (p < 0.05), with the exception of double-stance kneeling at 135 degrees of flexion. The resultant knee joint -reaction force increased with kneeling at all flexion angles. The compressive component of this force increased with kneeling for most conditions, while the lateral component of this force decreased significantly (p < 0.05) with kneeling only at 90 degrees , and the anterior component of this force increased significantly at all knee flexion angles (p < 0.05). Overall, kneeling had minimal changes on patellar tilt, with significant changes in patellar tilt seen only with kneeling at 120 degrees (p = 0.02 for double stance, and p = 0.03 for single stance).

CONCLUSIONS

The findings of this study suggest that kneeling at a higher flexion angle (135 degrees ) after total knee arthroplasty has a smaller effect on patellofemoral joint contact area and pressure than kneeling at lower flexion angles (<or=120 degrees ).

Knieprothesenkinematik

Until now it remains less clear to what extent the different types of endoprostheses can simulate the physiological motion pattern of the knee joint. The aim of this study was to present fluoroscopy and functional MRI as well as the results of these in vivo imaging techniques for TKA kinematics. Videofluoroscopy is a dynamic investigation, analyzing the subjects under fluoroscopic surveillance during different activities. Three-dimensional (3D) kinematics were recovered from the two-dimensional fluoroscopic images using a model-fitting technique. Kinematic analysis with functional MRI was performed in an open MR system at different flexion angles with external loads being applied during imaging. Femoropatellar and femorotibial 3D kinematics were analyzed by image postprocessing. The findings in healthy knees obtained with functional MRI under static conditions are in good agreement with the fluoroscopic outcome under dynamic conditions. In all investigated TKA in the mean an increased external rotated position of the femur relative to the tibia was observed at full extension, while the amount of external rotation during knee flexion was decreased. Although there was great variability among the individuals, differences were observed between the TKA-groups (e.g. posterior stabilized vs PCL retaining). Significant changes of femorotibial and femoropatellar kinematics were found in TKA compared to healthy knees, which may lead to early aseptic loosening or increased polyethylene wear The presented techniques and results allow for advanced in vivo diagnostics and may help to improve the design of TKA and to enhance the long-term performance.

Effects of patellar resurfacing on contact area and contact stress in total knee arthroplasty

The objective of this study was to examine the effects of patellar resurfacing on patellofemoral joint contact pressure and contact area in total knee arthroplasty. We tested seven fresh-frozen cadaveric knees using a custom knee jig which permits the simulation of physiologic quadriceps loading. Before patellar resurfacing, the mean peak contact pressure of medial and lateral patellofemoral joints was less than 10 MPa at knee flexion angles of 30 degrees, 60 degrees and 90 degrees, that of medial and lateral patellofemoral joints were 11.63 MPa and 11.42 MPa at a knee flexion angle of 120 degrees respectively, and the mean contact area of patellofemoral joint ranged from 70 to 150 mm2. After patellar resurfacing, the mean peak contact pressure of medial and lateral patellofemoral joints ranged from 50 to 100 Mpa (P<0.05), which exceeds the yield strength of ultrahigh molecular weight polyethylene, and the mean contact area of patellofemoral joint reduced to 10-15 mm2 (P<0.05). The contact pressure of patellofemoral joint was lower than the yield strength of articular cartilage before patellar resurfacing. Our results indicate that the yield stress of UHMWPE is exceeded after patellar resurfacing.

A new in vivo technique for determination of femoro-tibial and femoro-patellar 3D kinematics in total knee arthroplasty

Aim was to develop an in vivo technique which allows determination of femoro-tibial and of femoro-patellar 3D-kinematics in TKA simultaneously. The knees of 20 healthy volunteers and of eight patients with TKA (PCR, rotating platform) were investigated. Kinematics analysis was performed in an open MR-system at different flexion angles with external loads being applied. The TKA components were identified using a 3D-fitting technique, which allows an automated 3D-3D-registration of the TKA. Femoro-patellar and femoro-tibial 3D-kinematics were analyzed by image postprocessing. The validity of the postprocessing technique demonstrated a coefficient of determination of 0.98 for translation and of 0.97 for rotation. The reproducibility yielded a coefficient of variation (CV%) for patella kinematics between 0.17% (patello-femoral angle) and 6.8% (patella tilt). The femoro-tibial displacement also showed a high reproducibility with CV% of 4.0% for translation and of 7.1% for rotation. While in the healthy knees the typical screw-home mechanism was observed, a paradoxical anterior translation of the femur relative to the tibia combined with an external rotation occurred after TKA. Fifty percent of the TKA's experienced a condylar lift-off of >1mm predominately on the medial side. Regarding patellar kinematics significant changes were found in both planes in TKA with an increased patella height in the sagittal plane and patella tilt and shift in the transversal plane. The results demonstrate that the presented 3D MR-open based method is highly reproducible and valid for image acquisition and postprocessing and provides--for the first time--in vivo data of 3D-kinematics of the tibio-femoral and simultaneously of the patello-femoral joint during knee flexion.

Full arthroscopic lateral retinacular release with hook knife and quadriceps pressure-pull test: long-term follow-up

The most important causes of anterior knee pain include patellofemoral malalignment which causes patella-condyle contact anomalies at the patellofemoral joint, excessive patellar lateral pressure increase, trauma and overuse. In this article, besides presentation of late clinical results of 169 lateral retinacular release cases which were surgically treated between January 1995 and December 2002 with the help of a hook knife from the anterolateral portal due to lateral compression syndrome and patellar maltracking, we also described quadriceps tendon pressure-pull test which strongly indicates patellofemoral pain during physical examination of a patient with anterior knee pain. In addition to radiological patellofemoral imaging methods, we describe dynamic arthroscopic patellofemoral joint examination which is applied perarthroscopically to all of our surgically treated patients. We divided the patients into two groups: group 1 was the younger group with age 16-40 years; group 2 was the older group with age >41 years. Preoperative mean Lysholm scores for group I was 67.6 and 98.6, postoperatively, whereas it was 62.3 preoperatively and 91.4 postoperatively in group 2. This improvement of Lysholm scores postoperatively was statistically significant for each of the two groups (p=0.001). For group 1 preoperative IKDC scores were A in 8 patients, B in 61 patients and C in 11 patients, whereas it was A in 78 patients and B in 2 patients postoperatively. For group 2 preoperative IKDC scores were A in 2 patients, B in 43 patients, C in 36 patients and D in 8 patients, whereas it was A in 78 patients and B in 11 patients. This improvement of IKDC scores postoperatively was also statistically significant for each of the two groups (p=0.001). Preoperative and postoperative congruent angles of all patients were also measured. Preoperative mean congruent angle was +16.4 in group 1 while mean congruent angle was -7.1 postoperatively. For group 2 preoperative mean congruent angle was +18.7 preoperatively and -6.9 postoperatively. This improvement was statistically significant for each of the two groups (p=0.001). The overall number of patellar lateral compression syndrome cases were 51 (24 in group 1, 27 in group 2), patellar lateralization cases were 64 (28 in group 1, 36 in group 2) and patellar subluxation cases were 54 (28 in group 1, 26 in group 2). At the evaluation of arthroscopic lateral patellar facet and lateral femoral condylar chondral pathologies, we statistically showed that patellar chondral pathologies were more severe than the femoral chondral pathologies (p=0.001). In our opinion, the severity of patellar lateral facet chondral lesions, although it has a thicker layer of cartilage, is due to distribution of load to a larger contact area of lateral femoral condyle. Statistically increasing severity of femoral (Kendall's tau-b: 0.248, p=0.001) and patellar chondral lesions (Kendall's tau-b: 0.444, p=0.0001) with age is compatible with our arthroscopic and clinical observations. The most important complication seen in our cases was fibrosis at the site of lateral release, seen in three patients. They were healed without any sequela with local corticosteroid injection into the fibrosis tissue. We did not see any hemarthrosis or haematoma as a complication.

Contact stress at the post-cam mechanism in posterior-stabilised total knee arthroplasty

We measured the contact areas and contact stresses at the post-cam mechanism of a posterior-stabilised total knee arthroplasty when a posterior force of 500 N was applied to the Kirschner Performance, Scorpio Superflex, NexGen LPS Flex Fixed, and NexGen LPS Flex Mobile knee systems. Measurements were made at 90°, 120°, and 150° of flexion both in neutral rotation and 10° of internal rotation of the tibial component. Peak contact stresses at 90°, 120°, and 150° were 24.0, 33.9, and 28.8 MPa, respectively, for the Kirschner; 26.0, 32.4, and 22.1 MPa, respectively, for the Scorpio; and 34.1, 31.5, and 32.5 MPa, respectively, for the NexGen LPS Flex Fixed. With an internally rotated tibia, the contact stress increased significantly with all the fixed-bearing arthroplasties but not with the NexGen LPS Flex Mobile arthroplasty. The post-cam design should be modified in order to provide a larger contact area whilst avoiding any impingement and edge loading.

Accuracy of patellar resection in total knee replacement. A study using the medial pivot knee

A series of 80 patellae were randomly allocated to osteotomy by sawing or milling while implanting the medial pivot knee. Three landmarks were used to control the plane of the cut in the coronal plane. The lateral edge of the patellar tendon distally, and both medial and lateral edges of the quadriceps tendon proximally. A line drawn across the widest points if the patella (the patellar horizon) was used to analyse the slope of the cut (P angle) and the orientation of the patella relative to the trochlear groove both pre- and post-operatively (PF angle) on 45 degrees skyline views. No patellae were under resected. The mean thickness of the remnant was 16 mm (range 14-19). There was no difference between sawing and milling, but the former was technically preferable. No patellae were found to be subluxed. The mean P angle was 2 degrees (S.D. 3.2). The maximum P angle was 10 degrees , but in no cases did obliquity of cut with under resection of the medial patellar facet lead to lateral tilt of the patellar horizon. The mean post-operative PF angle was 2.3 degrees (S.D. 2.6). The majority of patellae retained a similar orientation to the patellar groove post-operatively. In three patients, the patella tilted laterally with respect to the groove. One resulted from under resection of the lateral patellar facet and two in which the slope of the cut was good, probably resulted from an error in femoral rotation. In all 80 knees, the patellar dome remained in full contact with the groove. Use of three fixed landmarks provides consistent patellar resection in terms of depth and slope. Errors in slope of up to 10 degrees do not cause patellar tilt but may lead to medial overload due to increasing thickness of the patella. Errors in femoral sizing and rotation are more potent causes of lateral tilt and overload.

Patella kinematics and patello-femoral contact areas in patients with genu varum and mild osteoarthritis

BACKGROUND

Patients with genu varum of the knee and moderate to severe osteoarthritis often suffer from additional symptoms of the patello-femoral joint. These patients have a poor prognosis following high tibial osteotomy. It is unclear whether varus knees with only mild femoro-tibial osteoarthritis are also associated with alterations of patella biomechanics, and affect the prognosis of intended high tibial osteotomy.

METHODS

Fifteen patients with genu varum and mild osteoarthritis and 15 healthy volunteers were assessed in an open MRI-scanner. 3D-GRE sequences of the knee were obtained in 0 degrees, 30 degrees and 90 degrees with and without activity of the extensor muscles. After segmentation of patella, femur, tibia and the adjacent cartilage, a patella-based local coordinate system was established. Femoral and tibial reference points allowed definition of the spatial position of the patella. Contact areas were defined by intersection of opposing cartilage volumes.

FINDINGS

No significant differences in patella kinematics and patello-femoral contact areas could be found (P > 0.05) between varus knees with mild osteoarthritis and healthy knees either at different flexion angles or under extending muscle activity.

INTERPRETATION

In knees with genu varum and mild medial osteoarthritis we could detect no alterations in patello-femoral kinematics. Since the alterations of patients with genu varum and mild osteoarthritis are restricted to the medial femoro-tibial joint high tibial osteotomy might be successful.

A new in vivo technique for determination of 3D kinematics and contact areas of the patello-femoral and tibio-femoral joint

Patello-femoral disorders are often caused by changes of patello-femoral and/or tibio-femoral kinematics. However, until now there has been no quantitative in vivo technique, that is able to obtain 3D kinematics and contact areas of all knee compartments simultaneously on a non-invasive basis. The aim of this study was therefore to develop and apply a technique which allows for determination of 3D kinematics and contact areas of the patello-femoral and tibio-femoral joint during different knee flexion angles and under neuromuscular activation patterns. One knee of each of the 10 healthy volunteers was examined in an open MR system under flexing isometric muscle activity at 30 degrees and 90 degrees. Three-dimensional kinematics and contact areas of the patello-femoral and tibio-femoral joints were analyzed by 3D image postprocessing. The reproducibility of the imaging technique yielded a coefficient of variation of 4.6% for patello-femoral, 4.7% for femoro-tibial displacement and 8.6% for contact areas. During knee flexion (30-90 degrees ), patella tilt (opened to medial) decreased (8.8+/-3.4 degrees vs. 4.6+/-3.1 degrees, p<0.05), while lateral patellar shift increased significantly (1.6+/-2.3mm vs. 3.4+/-3.0mm, p<0.05). Furthermore, a significant posterior translation and external rotation of the femur relative to the tibia was observed. Patello-femoral contact areas increased significantly in size (134+/-60mm(2) vs. 205+/-96 mm(2)) during knee flexion. This technique shows a high reproducibility and provides physiologic in vivo data of 3D kinematics and contact areas of the patello-femoral and the tibio-femoral joint during knee flexion. This allows for advanced in vivo diagnostics, and may help to improve therapy of patello-femoral disorders in the future.

Active patellar tracking measurement: a novel device using ultrasound

BACKGROUND

Many patients suffer patellar instability that may relate to transient patellar tracking abnormalities.

OBJECTIVE

To develop and test a technique to measure dynamic patellar tracking.

STUDY DESIGN

Controlled laboratory and in vivo study.

METHOD

A functional knee brace was modified to allow an ultrasound transducer to be mounted laterally to the femur, following the path of the patella during knee movement. An ultrasound system was used to measure patellar mediolateral position parallel to the femoral transepicondylar axis. Ten subjects with no patellar instability were studied to obtain patellar tracking and accuracy data.

RESULTS

The interobserver and intraobserver reproducibility ranged from 0.2 +/- 0.1 mm to 1.0 +/- 0.5 mm. The accuracy of the ultrasound measurement was checked against magnetic resonance imaging and was 0.6 +/- 1.9 mm. The patella moved medially then laterally from extension to flexion when sitting. Squatting and stepping produced a more lateral path, without the initial medial translation. The patella was more lateral during knee extension than during flexion.

CONCLUSIONS

This novel method for measurement of dynamic patellar mediolateral tracking was found to have good intraobserver and interobserver reproducibility, and the measurements matched closely with those obtained from magnetic resonance imaging reconstructions of static patellar positions. Some preliminary data for tracking in 3 activities were obtained from 10 normal knees.

Measuring the pressure pattern of the joint surface in the uninjured knee

PURPOSE

Testing an applicable intraoperative system for measuring surface pressure in knee joints, simulating as accurately as possible operating theatre conditions.

TYPE OF STUDY

Cadaver study.

METHODS

Pressure probes were introduced into the knee joints medially and laterally (K 6900 quad probes) in five cadavers, providing real-time data with computerized data recording (K-scan system, manuf. Tekscan Inc., South Boston, MA). The initial position of the knee was flexed and hanging, as in a leg holder. In simulation of usual operating theatre procedures, the knee was manually extended to 0 degrees and again brought to hanging position. The data are given as relative-pressure values and should serve as the basis for intraoperative use. During arthroscopy, absolute-pressure values would then be influenced by the pressure of the arthroscopy pump.

RESULTS

In 90 degrees flexion the average pressure ratio between the medial and lateral joint compartments was initially 1:1.5. When the leg was brought to full extension the pressure in the medial compartment increased, giving a pressure ratio of 1:1 at about 15 degrees and 1.8:1 at full extension. When bringing the leg back again to 90 degrees a similar pressure ratio curve was recorded.

CONCLUSIONS

The pressure relationship between the medial and lateral knee compartments could be recorded and was found reproducible in simulated operating-theatre conditions. The measurement of joint surface pressure during implantation of an anterior cruciate ligament graft could provide data for individual intraoperative quality control, thus improving surgical results.

In vivo assessment of patellofemoral joint contact area in individuals who are pain free

Magnetic resonance imaging was used to quantify in vivo patellofemoral joint contact area and to determine if contact area is affected by quadriceps muscle contraction. Ten subjects without pain (six women, four men) had their right patellofemoral joint imaged. Cartilage-enhanced, axial plane images were obtained at 0 degrees, 20 degrees, 40 degrees, and 60 degrees knee flexion under quadriceps loaded (contracted) and quadriceps unloaded (relaxed) conditions. Medial and lateral facet contact area measurements were obtained on each image, and then summed across all images in a series to yield facet contact area measurements for each knee angle. Total contact area was computed as the sum of medial and lateral facet contact areas. Consistent with in vitro studies, progressive increases in patellofemoral joint contact area were observed from 0 degrees to 60 degrees knee flexion. The lateral facet comprised a greater percentage of total contact area compared with the medial facet at each knee flexion angle, suggesting increased load-bearing potential. Quadriceps contraction did not affect patellofemoral joint contact area indicating that the addition of a compressive load to the joint did not alter the area of the load-bearing surfaces. In vivo assessment of patellofemoral joint contact area could provide insight into mechanisms of patellofemoral joint disorders.

A novel method for measuring medial compartment pressures within the knee joint in-vivo

A novel method for the measurement of knee joint forces in-vivo is described. A thin (0.2mm) flexible electronic pressure sensor was inserted through a narrow arthroscopic portal into the osteoarthritic medial compartment of the knee joint. The sensor partially covered the load bearing area. The surgery was performed under local anaesthetic during normal arthroscopic examination following patient consent. Results are presented for 11 patients. The method was used in a pilot study to assess the effects of four valgus knee braces on medial compartment forces. An analysis of variance could not detect un-loading by any brace although there were large variations in force output. These variations may be attributable to shifts in the sensor position. In-vivo measurement of joint force is technically feasible.

The effect of femoral component rotation and asymmetry in total knee replacements

Between April 1999 and February 2000, femoral component rotation and asymmetry (universal versus anatomic) were studied in a prospective randomized control trial in 101 consecutive patients undergoing total knee replacement. Patients were randomly assigned to four groups. The same surgical technique was used in all patients except for femoral component symmetry and rotational alignment. Universal femoral components provide a cheaper and equally reliable solution compared to anatomic compo nents. External rotation of the femoral component or using an anatomic femoral component did not statistically reduce the need for lateral release or improve patellar tracking.

Patellar tracking and patellofemoral geometry in deep knee flexion

Patellar tracking and femoral condylar geometry in deep knee flexion were evaluated using magnetic resonance imaging. The patellar tilting angle, patellar shift, and patellar anteroposterior translation from 0 degrees to 135 degrees flexion were measured. The depth of the femoral condylar articular surface and the curvature of the femoral condylar articular surface also were measured at 135 degrees flexion. The patella shifted laterally, tilted medially, and sank deeply into the intercondylar notch during deep knee flexion. The articular surface of the lateral condyle, existing deep within the intercondylar notch, began to curve steeply at a point farther from the center of the intercondylar notch than did the medial condyle. The geometry of the femoral condyle is adequate to fit the patellar geometry. Results of the current study suggest that the geometry of the lateral femoral condyle allows the patella to track smoothly with a larger patellofemoral contact area and less patellofemoral pressure during deep flexion.

Patellar component positioning in total knee arthroplasty

Five human anatomic specimen knees were used to determine the effect of patellar component position on patellofemoral kinematics, contact pressures, and contact areas after total knee arthroplasty using a polyethylene, domed patellar component. Each patellar component was positioned at the anatomic center of the resected patellar surface and then repositioned 5 mm proximally, distally, medially, and laterally. Patellar tilt was greatest with medial positioning of the patellar component and least with central and lateral positioning. At higher knee flexion angles, patellofemoral joint contact pressures increased at the medial facet with the medialized component and at the lateral facet for the lateralized component. The centralized component had the most evenly balanced patellar facet contact pressures. Distally positioned patellar components resulted in decreased patellar component loading at higher knee flexion angles. Central positioning of the patellar component results in optimal patellofemoral mechanics when maximal coverage of the resected patella is desired.

Eliminating patellofemoral complications in total knee arthroplasty: clinical and radiographic results of 121 consecutive cases using the Duracon system

This study reports the minimum 5-year follow-up of our experience with the Duracon Total Knee Arthroplasty System. A total of 121 consecutive total knee replacements using the Duracon system (Howmedica, Rutherford, NJ) were performed in 104 patients. Three patients died before the 5-year follow-up and were excluded from the final evaluation. The remaining 118 knees (101 patients) were assessed at a mean follow-up of 65 months (range, 60-80 months). The knee diagnoses were osteoarthritis in 97 patients, rheumatoid arthritis in 2 patients, osteonecrosis in 1 patient, and pigmented villonodular synovitis in 1 patient. The mean age was 70 years (range, 28-85 years). There were no reoperations for aseptic loosening, and there have been no reoperations for patellofemoral problems. At final follow-up evaluation, 112 knees (96%) had good or excellent results, and 6 knees (4%) had poor clinical results or went on to revision. For the surviving knees, the preoperative Knee Society objective score improved from a mean of 52 points (range, 20-72 points) to a final follow-up mean of 94 points (range, 66-100 points). Five knees needed reoperations: 2 knees in 1 patient because of acute hematogenous infection at 12 months, 1 knee because of a supracondylar femur fracture, 1 because of a patellar tendon rupture, and 1 to increase polyethylene thickness because of instability. The lack of aseptic loosening at the minimum 5-year follow-up compares favorably with any cemented or cementless series of knee replacement. The almost complete absence of patellofemoral complications in this series also indicates that the design changes, with particular attention to the trochlea design and patellofemoral contact throughout full flexion, have achieved their intended purpose. The results are encouraging at midterm, awaiting true long-term (15-20 years) follow-up.

Edge loading of patellar components after total knee arthroplasty

Patellofemoral joint kinematics, contact areas, contact pressures, and contact patterns were assessed after total knee arthroplasty (TKA) using human cadaver knees. Two contemporary TKA systems with anatomic patellofemoral joints were implanted and tested under anatomically based loading conditions. An electromagnetic tracking system was used to evaluate patellofemoral kinematics, and Fuji pressure-sensitive film was used to determine contact areas, pressures, and patterns. Edge loading of patellar components was observed at higher knee flexion angles with both TKA systems. Peak contact pressures seen at the regions of edge loading exceeded the yield strength of ultra-high-molecular weight polyethylene. Efforts to reduce edge loading and contact pressures may decrease the incidence of patellofemoral joint complications and component failure after TKA.