A stereophotogrammetric method for determining in situ contact areas in diarthrodial joints, and a comparison with other methods

Experimental Methods for Studying the Contact Mechanics of Joints

Biomechanics researchers often experimentally measure static or fluctuating dynamic contact forces, areas, and stresses at the interface of natural and artificial joints, including the shoulders, elbows, hips, and knees. This information helps explain joint contact mechanics, as well as mechanisms that may contribute to disease, damage, and degradation. Currently, the most common in vitro experimental technique involves a thin pressure-sensitive film inserted into the joint space; but, the film's finite thickness disturbs the joint's ordinary articulation. Similarly, the most common in vivo experimental technique uses video recording of 3D limb motion combined with dynamic analysis of a 3D link-segment model to calculate joint contact force, but this does not provide joint contact area or stress distribution. Moreover, many researchers may be unaware of older or newer alternative techniques that may be more suitable for their particular research application. Thus, this article surveys over 50 years of English-language scientific literature in order to (a) describe the basic working principles, advantages, and disadvantages of each technique, (b) examine the trends among the studies and methods, and (c) make recommendations for future directions. This article will hopefully inform biomechanics investigators about various in vitro and in vivo experimental methods for studying the contact mechanics of joints.

Toward defining the role of the synovium in mitigating normal articular cartilage wear and tear

Cartilage repair has been studied extensively in the context of injury and disease, but the joint's management of regular sub-injurious damage to cartilage, or 'wear and tear,' which occurs due to normal activity, is poorly understood. We hypothesize that this cartilage maintenance is mediated in part by cells derived from the synovium that migrate to the worn articular surface. Here, we demonstrate in vitro that the early steps required for such a process can occur. First, we show that under physiologic mechanical loads, chondrocyte death occurs in the cartilage superficial zone along with changes to the cartilage surface topography. Second, we show that synoviocytes are released from the synovial lining under physiologic loads and attach to worn cartilage. Third, we show that synoviocytes parachuted onto a simulated or native cartilage surface will modify their behavior. Specifically, we show that synoviocyte interactions with chondrocytes lead to changes in synoviocyte mechanosensitivity, and we demonstrate that cartilage-attached synoviocytes can express COL2A1, a hallmark of the chondrogenic phenotype. Our findings suggest that synoviocyte-mediated repair of cartilage 'wear and tear' as a component of joint homeostasis is feasible and is deserving of future study.

Empirical joint contact mechanics: A comprehensive review

Empirical joint contact mechanics measurement (EJCM; e.g. contact area or force, surface velocities) enables critical investigations of the relationship between changing joint mechanics and the impact on surface-to-surface interactions. In orthopedic biomechanics, understanding the changes to cartilage contact mechanics following joint pathology or aging is critical due to its suggested role in the increased risk of osteoarthritis (OA), which might be due to changed kinematics and kinetics that alter the contact patterns within a joint. This article reviews and discusses EJCM approaches that have been applied to articulating joints such that readers across different disciplines will be informed of the various measurement and analysis techniques used in this field. The approaches reviewed include classical measurement approaches (radiographic and sectioning, dye staining, casting, surface proximity, and pressure measurement), stereophotogrammetry/motion analysis, computed tomography (CT), magnetic resonance imaging (MRI), and high-speed videoradiography. Perspectives on approaches to advance this field of EJCM are provided, including the value of considering relative velocity in joints, tractional stress, quantification of joint contact area shape, consideration of normalization techniques, net response (superposition) of multiple input variables, and establishing linkages to regional cartilage health status. EJCM measures continue to provide insights to advance our understanding of cartilage health and degeneration and provide avenues to assess the efficacy and guide future directions of developing interventions (e.g. surgical, biological, rehabilitative) to optimize joint's health and function long term.

In vivo reverse total shoulder arthroplasty contact mechanics

BACKGROUND

Several in vitro studies have investigated the biomechanics of reverse total shoulder arthroplasty (RTSA); however, few in vivo studies exist. The purpose of this study was to examine in vivo RTSA contact mechanics in clinically relevant arm positions. Our hypothesis was that contact would preferentially occur in the inferior region of the polyethylene liner.

METHODS

Forty patients receiving a primary RTSA were recruited for a prospective cohort study. All patients received the same implant design with a nonretentive liner. Stereo radiographs were taken at maximal active range of motion. Model-based radiostereometric analysis was used to identify implant position. Contact area between the polyethylene and glenosphere was measured as the geometric intersection of the 2 components and compared with respect to polyethylene liner size, arm position, and relative position within the liner.

RESULTS

There were no differences in the proportion of contact area in any arm position between polyethylene liner sizes, ranging from 30% ± 17% to 38% ± 23% for 36-mm liners and 32% ± 21% to 41% ± 25% for 42-mm liners. Contact was equally distributed between the superior and inferior halves of the liner at each arm position (P = .06-.79); however, greater contact area was observed in the outer radius of the liner when the arm was flexed (P = .002).

CONCLUSION

This study highlights that contact mechanics are similar between 36- and 42-mm liners. Contact area is generally equally distributed throughout the liner across the range of motion and not preferentially in the inferior region as hypothesized.

Prediction of Individual Knee Kinematics From an MRI Representation of the Articular Surfaces

OBJECTIVE

The knowledge of individual joint motion may help to understand the articular physiology and to design better treatments and medical devices. Measurements of in-vivo individual motion are nowadays invasive/ionizing (fluoroscopy) or imprecise (skin markers). We propose a new approach to derive the individual knee natural motion from a three-dimensional representation of articular surfaces.

METHODS

We hypothesize that tissue adaptation shapes articular surfaces to optimize load distribution. Thus, the knee natural motion is obtained as the envelope of tibiofemoral positions and orientations that minimize peak contact pressure, i.e. that maximize joint congruence. We investigated four in-vitro and one in-vivo knees. Articular surfaces were reconstructed from a reference MRI. Natural motion was computed by congruence maximization and results were validated versus experimental data, acquired through bone implanted markers, in-vitro, and single-plane fluoroscopy, in-vivo.

RESULTS

In two cases, one of which in-vivo, maximum mean absolute error stays below 2.2° and 2.7 mm for rotations and translations, respectively. The remaining knees showed differences in joint internal rotation between the reference MRI and experimental motion at 0° flexion, possibly due to some laxity. The same difference is found in the model predictions, which, however, still replicate the individual knee motion.

CONCLUSION

The proposed approach allows the prediction of individual joint motion based on non-ionizing MRI data.

SIGNIFICANCE

This method may help to characterize healthy and, by comparison, pathological knee behavior. Moreover, it may provide an individual reference motion for the personalization of musculoskeletal models, opening the way to their clinical application.

Femoral Contact Forces in the Anterior Cruciate Ligament Deficient Knee: A Robotic Study

PURPOSE

To measure contact forces (CFs) at standardized locations representative of clinical articular cartilage defects on the medial and lateral femoral condyles during robotic tests with simulated weightbearing knee flexion.

METHODS

Eleven human knees had 20-mm-diameter cylinders of native bone/cartilage cored from both femoral condyles at standardized locations, with each cylinder attached to a custom-built load cell that maintained the plug in its precise anatomic position. A robotic test system was used to flex the knee from 0° to 50° under 200-N tibiofemoral compression without and with a 2 Nm internal tibial torque, 5 Nm external tibial torque, and 45 N anterior tibial force (AF). CFs and knee kinematics were recorded before and after cutting the anterior cruciate ligament (ACL).

RESULTS

ACL sectioning did not significantly increase medial or lateral CFs for any loading condition, with the exception of AF, in which increases in medial CF ranged from 38 N (at 15° flexion, P < .01) to 77 N (at 50° flexion, P < .002). Compared with the intact condition, ACL sectioning significantly increased anterior tibial translation by 12.33 mm (at 15° flexion, P < .001) and 17.4 mm (at 50° flexion, P < .001), and increased valgus rotation by 2.4° (at 15° flexion, P < .001) and 3.8° (at 50° flexion, P < .001).

CONCLUSIONS

Our hypothesis that CF would increase after ACL section was confirmed for the AF test condition only, and only for the medial condyle beyond 10° flexion. With the ACL sectioned, it appeared that the increased CF was owing to the medial condyle riding up over the posterior tibial plateau resulting from the large anterior tibial displacements.

CLINICAL RELEVANCE

Aside from our limited finding with AF, we concluded that CFs were generally unaffected by ACL section.

Biomechanical Measurement Error Can Be Caused by Fujifilm Thickness: A Theoretical, Experimental, and Computational Analysis

This is the first study to quantify the measurement error due to the physical thickness of Fujifilm for several material combinations relevant to orthopaedics. Theoretical and experimental analyses were conducted for cylinder-on-flat indentation over a series of forces (750 and 3000 N), cylinder diameters (0 to 80 mm), and material combinations (metal-on-metal, MOM; metal-on-polymer, MOP; metal-on-bone, MOB). For the scenario without Fujifilm, classic Hertzian theory predicted the true line-type contact width as W_O = {(8FD_cyl)/(πL_cyl)[(1 − ν_cyl²)/E_cyl + (1 − ν_flat²)/E_flat]}^1/2, where F is compressive force, D_cyl is cylinder diameter, L_cyl is cylinder length, ν_cyl and ν_flat are cylinder and flat Poisson's ratios, and E_cyl and E_flat are cylinder and flat elastic moduli. For the scenario with Fujifilm, experimental measurements resulted in contact widths of W_F = 0.1778 × F^0.2273 × D^0.2936 for MOM tests, W_F = 0.0449 × F^0.4664 × D^0.4201 for MOP tests, and W_F = 0.1647 × F^0.2397 × D^0.3394 for MOB tests, where F is compressive force and D is cylinder diameter. Fujifilm thickness error ratio W_F/W_O showed a nonlinear decrease versus cylinder diameter, whilst error graphs shifted down as force increased. Computational finite element analysis for several test cases agreed with theoretical and experimental data, respectively, to within 3.3% and 1.4%. Despite its wide use, Fujifilm's measurement errors must be kept in mind when employed in orthopaedic biomechanics research.

Introduction to Cartilage

Cartilage is a supporting connective tissue that, together with the bone, forms the framework supporting the body as a whole. There are many distinct types of cartilage, which exhibit numerous similarities as well as differences. Among them, articular cartilage is the best known and the most studied type. Articular cartilage is the thin layer of connective tissue that covers the articulating ends of bones in synovial (diarthrodial) joints. It provides a smooth surface for joint movement and acts as a load-bearing medium that protects the bone and distributes stress. The intense interest in articular cartilage is motivated by the critical role its degradation plays in arthritis and related joint diseases, which are the number one cause of disability in humans. This chapter discusses the physical, chemical and cellular properties of cartilage that give the tissue its extraordinary load-bearing characteristics.

Validation of a Finite Element Humeroradial Joint Model of Contact Pressure Using Fuji Pressure Sensitive Film

A finite element (FE) elbow model was developed to predict the contact stress and contact area of the native humeroradial joint. The model was validated using Fuji pressure sensitive film with cadaveric elbows for which axial loads of 50, 100, and 200 N were applied through the radial head. Maximum contact stresses ranged from 1.7 to 4.32 MPa by FE predictions and from 1.34 to 3.84 MPa by pressure sensitive film measurement while contact areas extended from 39.33 to 77.86 mm2 and 29.73 to 83.34 mm2 by FE prediction and experimental measurement, respectively. Measurements from cadaveric testing and FE predictions showed the same patterns in both the maximum contact stress and contact area, as another demonstration of agreement. While measured contact pressures and contact areas validated the FE predictions, computed maximum stresses and contact area tended to overestimate the maximum contact stress and contact area.

Improved Chondrogenic Capacity of Collagen Hydrogel-Expanded Chondrocytes: In Vitro and in Vivo Analyses

BACKGROUND

The use of autologous chondrocytes in cartilage repair is limited because of loss of the cartilage phenotype during expansion. The mechanosensing capacity of chondrocytes suggests evaluating the use of soft substrates for in vitro expansion. Our aim was to test the expansion of chondrocytes on collagen hydrogels to improve their capacity for chondrogenesis after a number of passages.

METHODS

Rat cartilage cells were expanded on collagen hydrogels and on plastic, and the preservation of their chondrogenic capacity was tested both in vitro and in vivo. The expression of relevant markers during expansion on each surface was measured by real-time PCR (polymerase chain reaction). Expanded cells were then implanted in focal lesions in the medial femoral condyle of healthy sheep, and the newly formed tissue was analyzed by histomorphometry.

RESULTS

Compared with cells cultured on plastic, cells cultured on hydrogels had better maintenance of the expression of the Sox9, Col2 (type-II collagen), FGFR3, and Alk-5 genes and decreased expression of Alk-1 and BMP-2. Pellets also showed increased expression of the cartilage marker genes aggrecan, Sox9, and Col2, and decreased expression of Col1 and Col10 (type-I and type-X collagen). ELISA (enzyme-linked immunosorbent assay) also showed a higher ratio of type-II to type-I collagen in pellets formed from cells expanded on hydrogels. When sheep chondrocytes were expanded and implanted in cartilage lesions in the femoral condyle of healthy sheep, hydrogel-expanded cells produced histologically better tissue compared with plastic-expanded cells.

CONCLUSIONS

The expansion of chondrocytes on collagen hydrogels yielded cells with an improved chondrogenic capacity compared with cells expanded on plastic.

CLINICAL RELEVANCE

The study results favor the use of hydrogel-expanded cells over the traditional plastic-expanded cells for autologous chondrocyte implantation.

Validation of an MRI-based method to assess patellofemoral joint contact areas in loaded knee flexion in vivo

PURPOSE

To develop and validate short axial and sagittal MRI scans (<1min) to assess in vivo patellofemoral contact areas in loaded knee flexion.

MATERIALS AND METHODS

Contact area was assessed in four cadaver knee specimens from axial and sagittal scans using two contact area extraction techniques (delineation and intersection) and three calculation techniques (slice thickness multiplication, linear interpolation, and spline interpolation). Error was expressed as the mean absolute and percentage difference from a dye staining-based reference standard. Intrareader and intrasubject repeatability, expressed as the mean standard deviation, was determined.

RESULTS

Contact area assessments from the sagittal MRI scans using the delineation and slice thickness multiplication technique had the smallest error (47.7 ± 38.1 mm(2) or 10.7%). The intrareader repeatability from assessments using the sagittal scans was smaller than those using the axial scans when the delineation method was used (<9.4 ± 4.3 mm(2) and <15.4 ± 14.1 mm(2) , respectively). The intrasubject repeatability of the assessment from the sagittal scan was less than 39.9 ± 23.0 mm(2) .

CONCLUSION

This protocol yields assessments of contact area in less than 1 minute that have errors similar to those made using scans many times longer and can be used in series with kinematic scans to carry out simultaneous assessments in vivo to study patellofemoral joint disease.

The measurement of joint mechanics and their role in osteoarthritis genesis and progression

Mechanics play a role in the initiation and progression of osteoarthritis. However, our understanding of which mechanical parameters are most important, and what their impact is on the disease, is limited by the challenge of measuring the most important mechanical quantities in living subjects. Consequently, comprehensive statements cannot be made about how mechanics should be modified to prevent, slow or arrest osteoarthritis. Our current understanding is based largely on studies of deviations from normal mechanics caused by malalignment, injury, and deformity. Some treatments for osteoarthritis focus on correcting mechanics, but there appears to be scope for more mechanically based interventions.

Response of sheep chondrocytes to changes in substrate stiffness from 2 to 20 Pa: effect of cell passaging

AIM

The influence of culture substrate stiffness (in the kPa range) on chondrocyte behavior has been described. Here we describe the response to variations in substrate stiffness in a soft range (2-20 Pa), as it may play a role in understanding cartilage physiopathology.

METHODS

We developed a system for cell culture in substrates with different elastic moduli using collagen hydrogels and evaluated chondrocytes after 2, 4, and 7 days in monolayer and three-dimensional (3D) cultures. Experiments were performed in normoxia and hypoxia in order to describe the effect of a low oxygen environment on chondrocytes. Finally, we also evaluated if dedifferentiated cells preserve the capacity for mechanosensing.

RESULTS

Chondrocytes showed less proliferating activity when cultured in monolayer in the more compliant substrates. Expression of the cartilage markers Aggrecan (Acan), type II collagen (Col2a1), and Sox9 was upregulated in the less stiff gels (both in monolayer and in 3D culture). Stiffer gels induced an organization of the actin cytoskeleton that correlated with the loss of a chondrocyte phenotype. When cells were cultured in hypoxia, we observed changes in the cellular response that were mediated by HIF-1α. Results in 3D hypoxia cultures were opposite to those found in normoxia, but remained unchanged in monolayer hypoxic experiments. Similar results were found for dedifferentiated cells.

CONCLUSIONS

Chondrocytes respond differently according to the stiffness of the substrate. This response depends greatly on the oxygen environment and on whether the chondrocyte is embedded or grown onto the hydrogel, since mechanosensing capacity was not lost with cell expansion.

Recent advances in computational mechanics of the human knee joint

Computational mechanics has been advanced in every area of orthopedic biomechanics. The objective of this paper is to provide a general review of the computational models used in the analysis of the mechanical function of the knee joint in different loading and pathological conditions. Major review articles published in related areas are summarized first. The constitutive models for soft tissues of the knee are briefly discussed to facilitate understanding the joint modeling. A detailed review of the tibiofemoral joint models is presented thereafter. The geometry reconstruction procedures as well as some critical issues in finite element modeling are also discussed. Computational modeling can be a reliable and effective method for the study of mechanical behavior of the knee joint, if the model is constructed correctly. Single-phase material models have been used to predict the instantaneous load response for the healthy knees and repaired joints, such as total and partial meniscectomies, ACL and PCL reconstructions, and joint replacements. Recently, poromechanical models accounting for fluid pressurization in soft tissues have been proposed to study the viscoelastic response of the healthy and impaired knee joints. While the constitutive modeling has been considerably advanced at the tissue level, many challenges still exist in applying a good material model to three-dimensional joint simulations. A complete model validation at the joint level seems impossible presently, because only simple data can be obtained experimentally. Therefore, model validation may be concentrated on the constitutive laws using multiple mechanical tests of the tissues. Extensive model verifications at the joint level are still crucial for the accuracy of the modeling.

Computational aspects in mechanical modeling of the articular cartilage tissue

This review focuses on the modeling of articular cartilage (at the tissue level), chondrocyte mechanobiology (at the cell level) and a combination of both in a multiscale computation scheme. The primary objective is to evaluate the advantages and disadvantages of conventional models implemented to study the mechanics of the articular cartilage tissue and chondrocytes. From monophasic material models as the simplest form to more complicated multiscale theories, these approaches have been frequently used to model articular cartilage and have contributed significantly to modeling joint mechanics, addressing and resolving numerous issues regarding cartilage mechanics and function. It should be noted that attentiveness is important when using different modeling approaches, as the choice of the model limits the applications available. In this review, we discuss the conventional models applicable to some of the mechanical aspects of articular cartilage such as lubrication, swelling pressure and chondrocyte mechanics and address some of the issues associated with the current modeling approaches. We then suggest future pathways for a more realistic modeling strategy as applied for the simulation of the mechanics of the cartilage tissue using multiscale and parallelized finite element method.

Identification of signalling pathways triggered by changes in the mechanical environment in rat chondrocytes

AIM

The aim of this work was to determine the pathways implicated in the mechanosensing of chondrocytes.

METHODS

Rat chondrocytes were cultured in collagen hydrogels of different stiffness (2-20 Pa) in normoxia and hypoxia, in monolayer and embedded inside hydrogels. First, chondrocyte were cultured on hydrogels in the presence of antibodies to block integrins. Second, custom RT-PCR array plates and western blot were used to detect changes in expression of genes implicated in downstream signalling pathways.

RESULTS

The results allowed us to demonstrate the mechanosensing of chondrocytes for changes in stiffness in the range of Pascals. We also identified Non-Muscle Myosin II (NMMII) and integrins α1, β1 and β3 as participants in the mechanosensing, since their blockade inhibits the sensing of the stiffness, and they are up-regulated in the process. RT-PCR arrays and western blot detected up-regulation of Paxillin, RhoA, Fos, Jun and Sox9. We detected no expression of Src in the monolayer cultures, but we found a role for this protein in 3D. The expression of HIF-1α was not modified under normoxia but was found to participate under hypoxia. Focal Adhesion Kinase (FAK), showed a direct relationship with the expression of Aggrecan in hypoxia and an inverse one in normoxia. Finally, immunofluorescence analysis located the expression of factors AP-1, Sox-9 and HIF-1α inside the cell nuclei and RhoA, Src, Paxillin and FAK close to the cytoplasmic membrane.

CONCLUSIONS

We determined here some of the genes that are up-regulated during the process of chondrocyte mechanosensing.

In vivo shoulder function after surgical repair of a torn rotator cuff: glenohumeral joint mechanics, shoulder strength, clinical outcomes, and their interaction

BACKGROUND

Surgical repair of a torn rotator cuff is based on the belief that repairing the tear is necessary to restore normal glenohumeral joint (GHJ) mechanics and achieve a satisfactory clinical outcome.

HYPOTHESIS

Dynamic joint function is not completely restored by rotator cuff repair, thus compromising shoulder function and potentially leading to long-term disability.

STUDY DESIGN

Controlled laboratory study and Case series; Level of evidence, 4.

METHODS

Twenty-one rotator cuff patients and 35 control participants enrolled in the study. Biplane radiographic images were acquired bilaterally from each patient during coronal-plane abduction. Rotator cuff patients were tested at 3, 12, and 24 months after repair of a supraspinatus tendon tear. Control participants were tested once. Glenohumeral joint kinematics and joint contact patterns were accurately determined from the biplane radiographic images. Isometric shoulder strength and patient-reported outcomes were measured at each time point. Ultrasound imaging assessed rotator cuff integrity at 24 months after surgery.

RESULTS

Twenty of 21 rotator cuff repairs appeared intact at 24 months after surgery. The humerus of the patients' repaired shoulder was positioned more superiorly on the glenoid than both the patients' contralateral shoulder and the dominant shoulder of control participants. Patient-reported outcomes improved significantly over time. Shoulder strength also increased over time, although strength deficits persisted at 24 months for most patients. Changes over time in GHJ mechanics were not detected for either the rotator cuff patients' repaired or contralateral shoulders. Clinical outcome was associated with shoulder strength but not GHJ mechanics.

CONCLUSION

Surgical repair of an isolated supraspinatus tear may be sufficient to keep the torn rotator cuff intact and achieve satisfactory patient-reported outcomes, but GHJ mechanics and shoulder strength are not fully restored with current repair techniques.

CLINICAL RELEVANCE

The study suggests that current surgical repair techniques may be effective for reducing pain but have not yet been optimized for restoring long-term shoulder function.

Importance of the dome and posterior wall as evidenced by bone density mapping in the acetabulum

BACKGROUND

Characterizing the distribution of bone density in the acetabulum is of importance in better understanding and guiding treatment for both osteoarthritis and trauma of the hip joint. This study aims to develop a highly automated method to quantify the pattern of subchondral bone density in the acetabulum using clinically identifiable regions.

METHODS

Subchondral acetabular bone density distribution maps were created bilaterally from 30 non-pathologic pelvic CT scans. The density maps were aligned orthogonal to the acetabular rim plane and divided into twelve zones. Average bone density was calculated in each of these zones and compared to investigate differences between regions within each acetabulum and between right and left sides in a given patient.

FINDINGS

In all patients, the zones corresponding to the dome and posterior wall of the acetabulum demonstrated significantly higher average bone densities than all other regions (P<0.01). Significant correlations (R=0.4 to 0.76, P<0.05) were found between corresponding regions of the left and right sides in 10 of the 12 zones.

INTERPRETATION

The pattern of subchondral bone density distribution found in this study is consistent with previously observed bone density patterns in the acetabulum. Correspondence of right and left sides suggests that the distribution of loading on the acetabulum is similar on both sides in healthy individuals, though differences may exist in load sharing. Quantifying bone density patterns using zonal density map analysis may lead to a better understanding of the impact of traumatic injuries and progression of pathologic conditions in the hip joint.

Comparative 3D quantitative analyses of trapeziometacarpal joint surface curvatures among living catarrhines and fossil hominins

Comparisons of joint surface curvature at the base of the thumb have long been made to discern differences among living and fossil primates in functional capabilities of the hand. However, the complex shape of this joint makes it difficult to quantify differences among taxa. The purpose of this study is to determine whether significant differences in curvature exist among selected catarrhine genera and to compare these genera with hominin fossils in trapeziometacarpal curvature. Two 3D approaches are used to quantify curvatures of the trapezial and metacarpal joint surfaces: (1) stereophotogrammetry with nonuniform rational B-spline (NURBS) calculation of joint curvature to compare modern humans with captive chimpanzees and (2) laser scanning with a quadric-based calculation of curvature to compare modern humans and wild-caught Pan, Gorilla, Pongo, and Papio. Both approaches show that Homo has significantly lower curvature of the joint surfaces than does Pan. The second approach shows that Gorilla has significantly more curvature than modern humans, while Pongo overlaps with humans and African apes. The surfaces in Papio are more cylindrical and flatter than in Homo. Australopithecus afarensis resembles African apes more than modern humans in curvatures, whereas the Homo habilis trapezial metacarpal surface is flatter than in all genera except Papio. Neandertals fall at one end of the modern human range of variation, with smaller dorsovolar curvature. Modern human topography appears to be derived relative to great apes and Australopithecus and contributes to the distinctive human morphology that facilitates forceful precision and power gripping, fundamental to human manipulative activities.

Imaging the role of biomechanics in osteoarthritis

Osteoarthritis is widely believed to result from local mechanical factors acting within the context of systemic susceptibility. This narrative review delineates current understanding of the etiopathogenesis of osteoarthritis and more specifically examines the critical role of biomechanics in disease pathogenesis. There are several ways the mechanical forces across the joint can be measured, including some that rely heavily on imaging methods. These are described and methods to advance the field are proposed.

In Vivo Measurement of Glenohumeral Joint Contact Patterns

The objectives of this study were to describe a technique for measuring in-vivo glenohumeral joint contact patterns during dynamic activities and to demonstrate application of this technique. The experimental technique calculated joint contact patterns by combining CT-based 3D bone models with joint motion data that were accurately measured from biplane x-ray images. Joint contact patterns were calculated for the repaired and contralateral shoulders of 20 patients who had undergone rotator cuff repair. Significant differences in joint contact patterns were detected due to abduction angle and shoulder condition (i.e., repaired versus contralateral). Abduction angle had a significant effect on the superior/inferior contact center position, with the average joint contact center of the repaired shoulder 12.1% higher on the glenoid than the contralateral shoulder. This technique provides clinically relevant information by calculating in-vivo joint contact patterns during dynamic conditions and overcomes many limitations associated with conventional techniques for quantifying joint mechanics.

[Determination of joint contact area using MRI]

Elevated contact stress on the articular joints has been hypothesized to contribute to articular cartilage wear and joint pain. However, given the limitations of using contact stress and areas from human cadaver specimens to estimate articular joint stress, there is need for an in vivo method to obtain such data. Magnetic resonance imaging (MRI) has been shown to be a valid method of quantifying the human joint contact area, indicating the potential for in vivo assessment. The purpose of this study was to describe a method of quantifying the tibiofemoral joint contact area using MRI. The validity of this technique was established in porcine cadaver specimens by comparing the contact area obtained from MRI with the contact area obtained using pressure-sensitive film (PSF). In particular, we assessed the actual condition of contact by using the ratio of signal intensity of MR images of cartilage surfaces. Two fresh porcine cadaver knees were used. A custom loading apparatus was designed to apply a compressive load to the tibiofemoral joint. We measured the contact area by using MRI and PSF methods. When the ratio of signal intensity of the cartilage surface was 0.9, the error of the contact area between the MR image and PSF was about 6%. These results suggest that this MRI method may be a valuable tool in quantifying joint contact area in vivo.

The measurement of joint mechanics and their role in osteoarthritis genesis and progression

Mechanics play a role in the initiation, progression, and successful treatment of osteoarthritis. However, we don't yet know enough about which specific mechanical parameters are most important and what their impact is on the disease process to make comprehensive statements about how mechanics should be modified to prevent, slow, or arrest the disease process. The objectives of this review are (1) to summarize methods for assessing joint mechanics and their relative merits and limitations, (2) to describe current evidence for the role of mechanics in osteoarthritis initiation and progression, and (3) to describe some current treatment approaches that focus on modifying joint mechanics.

Intra-articular compressive stress of the elbow joint in extension: an experimental study using Fuji films

The use of Fuji films is simple but their manipulation and result interpretation seem to be difficult in the framework of medical research. The reliability and reproducibility of Fuji films have been proved by many previous studies. This study was undertaken to know precisely the articular zones of the elbow and to determine the compressive stress these areas undergo during different activities, in order to assess the importance of different articular contact areas. These data indicate the need for better-adapted elbow prosthesis and can be eventually used to design more durable prosthesis for the elbow. The compressive stress on the radial head was less than 25% in extension. The stress on the radial head varied from the neutral position (23% of the stress), to full pronation (11% of the stress) and to full supination (6% of the stress). The Humero-ulnar compartment had the maximum impact. Coronoid process seemed to be a fundamental element of the elbow joint in extension (60% of total compressive stress). The Medial humero-ulnar compartment was less stressed than the lateral compartment. The radial head does not seem to play a major role in the stability of the elbow in extension if the ulnar collateral ligament exists. The ulnar collateral ligament is essential to the elbow joint stability. The lifespan of a non-constrained prosthesis would depend on the existence of the couple: radial head/ulnar collateral ligament; the absence of radial head could compromise the humero-ulnar stability. This work paved the way for the designing of new non-constrained elbow prosthesis with the reconstruction of the radial head.

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 ).

A stereophotographic study of ankle joint contact area

The purpose of this study was to measure the ankle joint contact area under physiological load magnitudes using a stereophotography technique that allows accurate analysis of the entire joint surface without disrupting the joint during loading. Ten cadaveric foot and ankle specimens were loaded to 1000 N in neutral, and 20 degrees dorsiflexion, supination, pronation, and plantarflexion. Photo targets rigidly fixed to each of the bones were imaged in the loaded joint position using a high-resolution stereophotography system. After testing, each ankle was disarticulated and the joint surfaces imaged relative to the photo targets. The photo targets were then used to spatially register the joint surfaces into the loaded joint position; the overlap of the surfaces was used to determine the joint contact area. The mean talo-tibia contact area was greatest in dorsiflexion 7.34 +/- 1.69 cm(2) and was significantly larger than in plantar flexion (p < 0.05), which showed the smallest joint contact area 4.39 +/- 1.41 cm(2). Considering talo-fibula, the maximum contact area was measured in dorsiflexion, 2.02 +/- 0.78 cm(2), and the minimum contact area occurred in pronation, 0.77 +/- 0.49 cm(2), respectively (p < 0.05). The reported stereophotography technique allows measurement of the joint contact area without disrupting the joint during loading. The contact area is larger than previously reported, as the entire joint surface was analyzed. Joint contact extends over both the talar dome and the talar shoulders where osteochondritis dissecans lesions commonly occur.

In vivo contact pressures in total knee arthroplasty

This study compared the in vivo femoropolyethylene contact pressures generated in fixed-bearing total knee arthroplasty (TKAs) with those in mobile-bearing TKAs. In vivo kinematics obtained from a 2-dimensional to 3-dimensional registration technique and soft tissue locations derived from computed tomographic scans were entered into a 3-dimensional inverse dynamics mathematical model to determine the in vivo bearing contact forces. The contact areas were obtained from the assembly of computer-aided design models of the components. The contact pressure was defined as the ratio of the contact forces to the contact areas. The results indicate that the in vivo contact pressures in each TKA are greater for the medial condyle than for the lateral condyle. The ability of the mobile-bearing TKA to rotate maintains higher femoropolyethylene contact, resulting in lesser contact pressures, as compared with the fixed-bearing TKA.

MRI-based modeling for evaluation of in vivo contact mechanics in the human wrist during active light grasp

Investigations of in vivo joint mechanics are important for understanding the joint function under functional loading and the mechanisms of pathology. In this study we used magnetic resonance imaging (MRI) based joint contact modeling to evaluate in vivo joint contact mechanics in the human wrist. MRI scans were performed on the wrists of four subjects while they maintained light grasp of a cylinder, and with the same wrist relaxed. 3D models of the radius, scaphoid and lunate, including cartilage surface data, were constructed from the relaxed image data. These models were transformed into the loaded configuration, as determined from the grasp image data, and contact mechanics were evaluated. The resulting contact pressures, areas and forces were then analyzed for each articulation and for each subject. Contact areas were measured directly from grasp MRI images for comparison to the model predictions. The first-ever estimates for in vivo radioscaphoid and radiolunate contact pressure agreed reasonably well with previous cadaveric studies. This investigation also produced novel in vivo scapholunate contact results that were similar to radiolunate data. The specimen-specific contact area comparison generally showed substantial variability between the models and the direct measurements from MRI. On average, the models were within about 10% of the direct MRI measurements for radioscaphoid and scapholunate contact areas, but radiolunate contact areas from the model were only within 55% of the direct measurements. Overall, the results of the study suggest that MRI-based modeling has substantial potential for evaluation of in vivo joint contact mechanics, especially as technology and methodology improve.

Relative positions of the contacts on the cartilage surfaces of the knee joint

The goal of this project was to determine the centers of contacts (points of closest approach of the articular surfaces) for the tibio-femoral and patello-femoral joints throughout the flexion range, with a focus on high flexion where there is potential overlap between the contacts. The purpose was to determine the implications to the design of joint replacements and tissue engineered implants. Eight cadaveric knees were mounted in a rig with different loading combinations applied to the femur, including axial load, anterior/posterior shear, and internal/external torque. The range of flexion was 0 degrees to 155 degrees . Reference points on the bones measured during the experiments were used to later reconstruct an accurate 3D computer model of the multiple joint positions and determine the centers of contact between the opposing bearing surfaces. The tibio-femoral contact at 0 degrees flexion was displaced 5 mm anterior to the notch (the end point of the articular cartilage on the mid-line of the femoral sulcus) on the medial side, while remaining level with the notch on the lateral side. The patella contacts on the femur extended 15 mm posterior to the intercondylar notch with a centerline between the lateral and medial paths being several millimeters lateral to the center of the femur. The centers of the patella contacts were close to the inner margin of the medial condyle and did not directly overlap with the centers of the tibial contacts. On the lateral side the patella contacts ended where the tibial contacts began. For applications to medial unicondylar knee replacement design it was shown that patellar impingement on the anterior of the femoral component would occur at 110 degrees flexion. For TKR design, a continuous patella contact up to high flexion could be obtained by extending the trochlea 15 mm posterior to the intercondylar notch.

Measuring contact area, force, and pressure for bioengineering applications: Using Fuji Film and TekScan systems

The goal of this study was to compare the TekScan I-Scan Pressure Measurement System with two methods of analysis involving the Fuji Film Prescale Pressure Measuring System in estimating area, force and pressure. Fuji Film and TekScan sensors were alternately placed between a cylindrical peg and a finely ground steel base plate, and compressed with known forces. All Fuji stains were digitally scanned and analyzed. The Erase method of Fuji Film analysis consisted of manually removing portions of the image judged by the user to be outside the perimeter of the stain. The second method of Fuji Film analysis, termed the Threshold method, used the threshold tool to analyze only those pixels that were stained from loading. The TekScan system utilized special matrix-based sensors interfaced with a Windows compatible desktop computer that was equipped with specialized data acquisition hardware and analysis software. The data from this study did not support the hypothesis that all three methods would have accuracies within +/-5% of a known value, when estimating area, force and pressure. Specifically, the TekScan system was found to be more accurate than either of the Fuji Film methods when estimating area and pressure.

An in vitro analysis of ligament reconstruction or extension osteotomy on trapeziometacarpal joint stability and contact area

PURPOSE

Painful instability of the minimally osteoarthritic thumb carpometacarpal (CMC) joint can be treated successfully by either ligament reconstruction or metacarpal extension osteotomy. The purpose of this study was to measure the laxity of cadaveric thumb CMC joints and to determine the influence of ligament reconstruction and metacarpal osteotomy on joint laxity and contact area.

METHODS

The baseline laxity of CMC joints from 25 fresh-frozen human cadaveric specimens (average age, 42 y; range, 18-55 y) was measured in the position of lateral pinch on a custom-designed CMC joint laxity tester. Joint laxity was measured again after 2 surgical simulations consisting of either a metacarpal extension osteotomy (at 10 degrees and 15 degrees) or a simulated Eaton-Littler ligament reconstruction (including total, volar, and dorsal ligament reconstructions relative to the plane of the thumbnail). Contact area between the thumb metacarpal and trapezium during testing was determined using stereophotogrammetry.

RESULTS

The 15 degrees extension osteotomy significantly reduced CMC joint laxity in the radial-ulnar, dorsal-volar, pronation-supination, and distraction directions in the position of lateral pinch. The 10 degrees osteotomy reduced laxity only in the dorsal-volar direction. The total ligament reconstruction significantly reduced joint laxity in the radial-ulnar, dorsal-volar, and pronation-supination directions. The dorsal ligament reconstruction reduced laxity in the dorsal-volar direction only; the volar ligament reconstruction reduced laxity in both dorsovolar and radioulnar directions. The 10 degrees and 15 degrees osteotomies produced a dorsal shift of the weighted centroid of contact on the metacarpal and trapezium, whereas the ligament reconstruction did not produce such an effect.

CONCLUSIONS

In the position of lateral pinch the 15 degrees osteotomy and total ligament reconstruction significantly reduced laxity of the thumb CMC joint in all directions tested. The isolated dorsal or volar ligament reconstructions both reduced dorsal-volar laxity. Metacarpal extension osteotomy may stabilize the thumb CMC joint in lateral pinch to a degree similar to that of a standard ligament reconstruction.

Biomechanics of the knee joint in flexion under various quadriceps forces

Bioemchanics of the entire knee joint including tibiofemoral and patellofemoral joints were investigated at different flexion angles (0 degrees to 90 degrees ) and quadriceps forces (3, 137, and 411 N). In particular, the effect of changes in location and magnitude of restraining force that counterbalances the isometric extensor moment on predictions was investigated. The model consisted of three bony structures and their articular cartilage layers, menisci, principal ligaments, patellar tendon, and quadriceps muscle. Quadriceps forces significantly increased the anterior cruciate ligament, patellar tendon, and contact forces/areas as well as the joint resistant moment. Joint flexion, however, substantially diminished them all with the exception of the patellofemoral contact force/area that markedly increased in flexion. When resisting extensor moment by a force applied on the tibia, the force in cruciate ligaments and tibial translation significantly altered as a function of magnitude and location of the restraining force. Quadriceps activation generated large ACL forces at full extension suggesting that post ACL reconstruction exercises should avoid large quadriceps exertions at near full extension angles. In isometric extension exercises against a force on the tibia, larger restraining force and its more proximal location to the joint substantially decreased forces in the anterior cruciate ligament at small flexion angles whereas they significantly increased forces in the posterior cruciate ligament at larger flexion angles.

In vivo contact areas of the knee in patients with patellar subluxation

OBJECTIVE

Ex vivo studies have suggested that cartilage contact areas and pressure are of high clinical relevance in the etiology of osteoarthritis in patients with patellar subluxation. The aims of this study were therefore to validate in vivo measurements of contact areas with 3D open magnetic resonance imaging (MRI), and to study knee joint contact areas in patients with patellar subluxation at different angles of knee flexion in comparison with healthy subjects.

METHODS

3D-MRI data sets of 12 healthy volunteers and eight patients with patellar subluxation were acquired using a standard clinical (1.5 T) and an open (0.2 T) MRI scanner. We compared femoro-patellar and femoro-tibial contact areas obtained with two different sequences from open MRI [dual-echo-steady-state (DESS) and fast-low-angle-shot (FLASH) sequences] with those derived from standard clinical 1.5 T MRI. We then analyzed differences in joint contact areas between healthy subjects and patients with patellar subluxation at 0 degree, 30 degrees, and 90 degrees of knee flexion using open MRI.

RESULTS

The correlation of the size of contact areas from open MRI with standard clinical MRI data ranged from r = 0.52 to 0.92. Open-MRI DESS displayed a smaller overestimation of joint contact areas (+21% in the femoro-patellar, +12% in the medial femoro-tibial, and +19% in the lateral femoro-tibial compartment) than FLASH (+40%, +37%, +30%, respectively). The femoro-patellar contact areas in patients were significantly reduced in comparison with healthy subjects (-47% at 0 degree, -56% at 30 degrees, and -42% at 90 degrees of flexion; all p < 0.01), whereas no significant difference was observed in femoro-tibial contact areas.

CONCLUSIONS

Open MRI allows one to quantify joint contact areas of the knee with reasonable accuracy, if an adequate pulse sequence is applied. The technique permits one to clearly identify differences between patients with patellar subluxation and healthy subjects at different flexion angles, demonstrating a significant reduction and lateralization of contact areas in patients. In the future, application of this in vivo technique is of particular interest for monitoring the efficacy of different types of surgical and conservative treatment options for patellar subluxation.

In vivo tibiofemoral contact analysis using 3D MRI-based knee models

This paper quantified the motion of the tibiofemoral contact points during in vivo weight bearing flexion using MRI- based 3D knee models and two orthogonal fluoroscopic images. The contact points on the medial and lateral tibial plateau were calculated by finding the centroid of the intersection of the tibial and femoral cartilage layers and by using the bony geometry alone. Our results indicate that the medial femoral condyle remains in the central portion of the tibial plateau and the lateral condyle translates posteriorly with increasing flexion. Using the bony contact model increased the total translation of the medial and lateral condyles by 250 and 55%, respectively, compared to the cartilage contact model. These results suggest that using the bony geometry alone may not accurately represent the articular surfaces of the knee. Articular cartilage geometry may have to be used to accurately quantify tibiofemoral contact.

Contact stresses in the knee joint in deep flexion

The contact stresses in the knee that arise from activities involving deep flexion have not been given due consideration in view of social and cultural practice amongst many Asians that frequently cause the engagement of these activities. Excessively large stresses (>25 MPa) can cause cartilage damage and may be the precursor to the development of degenerative disease of the joint. In this study, forces in the knee derived from previous studies of human walking and squatting were applied to five cadaver knees that underwent quasi-static mechanical testing. This was conducted using a materials-testing machine and a custom-made apparatus that allowed secure and consistent loading of the knee specimen in flexion beyond 120 degrees. A thin-film electronic pressure transducer was inserted into the cadaver tibiofemoral joint space to measure force and area. Throughout the various positions simulating specific phases of walking, it was found that stresses peaked to 14 MPa (standard deviation was 2.5 MPa). In deep flexion, the peak stresses were significantly larger by over 80%, reaching the damage limits of cartilage. The results from this biomechanical study suggest that the adequacy of articular cartilage to support loads in the knee joint during deep flexion may be questionable.

Biomechanical evaluation of a novel glenoid design in total shoulder arthroplasty

The amount of articular curvature mismatch providing optimal performance in total shoulder arthroplasty (TSA) is unknown. The objectives of this study were to quantify glenohumeral joint mechanics before and after TSA and to compare the performance of 3 glenoid components: (1) nonconforming, (2) conforming, and (3) a novel design featuring a conforming center extending into a nonconforming periphery. Six fresh-frozen cadaveric shoulders (mean age, 43 years) were mechanically tested on a custom apparatus by use of simulated muscle forces and a coordinate-measuring machine to determine joint kinematics. B-spline models of the natural and prosthetic articular surfaces were generated, and joint contact was computed by use of a proximity criterion. During both centered ( P = .02) and eccentric ( P = .05) loading protocols, glenoid contact migrated posteriorly in conforming implants. No statistical differences in kinematics and contact were found among the nonconforming design, the new design, and the natural joint. Therefore, adding a central region of conformity does not compromise the ability of nonconforming TSA components to reduce rim loading.

In vitro force mapping of normal canine humeroradial and humeroulnar joints

OBJECTIVE

To determine the distribution of force between the articular surfaces of the humerus and radius and between the humerus and ulna in normal canine forelimbs.

SAMPLE POPULATION

12 cadaveric canine right forelimbs.

PROCEDURE

Transarticular force maps were created by placing a tactile array pressure sensor into the elbow joint cavity and loading cadaveric forelimbs in a materials testing system. Mean joint forces were determined at loads of 50, 100, 150, and 200 N.

RESULTS

All tests produced 2 distinct areas of high load that corresponded with the proximal articular surfaces of the radius and ulna. Mean forces for the radial proximal articular surface were slightly but significantly greater than for the ulna, averaging 51% to 52% of total force for all applied loads.

CONCLUSIONS AND CLINICAL RELEVANCE

The proximal articular surface of the ulna contributes substantially to load transfer through the canine elbow joint. Abnormalities, which increase this load, might contribute to canine elbow joint dysplasia, specifically fragmentation of the medial coronoid process and osteochondritis dissecans of the medial aspect of the humeral condyle. In the treatment of these conditions, the normal force distribution within the canine elbow joint should be taken into consideration.

Positional-dependent changes in glenohumeral joint contact pressure and force: possible biomechanical etiology of posterior glenoid wear

The purpose of this study was to simulate the forces in the individual periscapular muscles and investigate the possible biomechanical etiology of posterior erosion of the glenoid. Twelve fresh-frozen human cadaveric shoulders were used with a custom shoulder jig that uses independently controlled pneumatic cylinders to apply forces across the tendon of each muscle. The simulated muscle forces included the rotator cuff, the deltoid, the latissimus dorsi, and the pectoralis major. The shoulders were tested in 12 overhead activity positions. A 6-degree-of-freedom load cell was used to measure the glenohumeral joint forces, and the glenohumeral contact pressures and areas were measured by use of Fuji pressure-sensitive film. There were no significant differences in glenohumeral joint forces between 60 degrees and 90 degrees of vertical abduction or between 60 degrees and 90 degrees of external rotation. At 70 degrees of horizontal adduction from the scapular plane, there was a significant decrease in superior force, a significant increase in posteriorly directed force, and a significant decrease in compression when compared with 30 degrees and 50 degrees of horizontal adduction ( P < .05). Similar statistical trends were seen for 60 degrees of vertical adduction. For the glenohumeral contact area and pressure, a significant decrease in contact area was seen between 30 degrees of horizontal adduction and 70 degrees of horizontal adduction for both 60 degrees and 90 degrees of vertical adduction ( P < .05). A significant increase in contact pressure was seen at 70 degrees of horizontal adduction compared with 50 degrees and 30 degrees ( P < .05). The contribution of the humerothoracic muscles is significant and should be considered for proper restoration of glenohumeral joint biomechanics. Furthermore, asymmetric loading with excessive or repetitive overhead activities may eventually lead to posterior glenoid erosion.

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 three-dimensional MRI analysis of knee kinematics

PURPOSE

To quantify normal, in vivo tibio-femoral knee joint kinematics in multiple weight bearing positions using non-invasive, high-resolution MRI and discuss the potential of developing future kinematic methods to assess patients with abnormal joint pathologies.

METHODS

Ten volunteers with clinically normal knees pushed inferiorly on the footplate of a weight bearing apparatus inside the MR scanner. The volunteers held the weight (133 N) for five scans as the knee motion was evaluated from 0 degrees to 60 degrees of flexion. Full extension was set as the zero point for all measured parameters. Using 3D reconstructions, tibia motion relative to the femur and flexion angle was measured as varus-valgus angle, axial rotation, anterior-posterior translation, and medial-lateral translation. Medial and lateral compartment tibio-femoral contact areas were examined and centroids of the contract areas were calculated.

RESULTS

Tibial internal rotation averaged 4.8 degrees at 40 degrees of flexion and then decreased. Tibial valgus increased by 8 degrees at 60 degrees of flexion. Femoral roll back also increased to 18.5 mm average at 60 degrees of flexion, while the tibia translated medially 2.5 mm. Medial compartment femoro-tibial contact area started at 374 mm2 and decreased to 308 mm2 with flexion of 60 degrees, while lateral compartment contact area did not change significantly from 276 mm2.

CONCLUSIONS

Results correlate with previous studies of knee kinematics while providing greater three-dimensional detail. MR imaging allows excellent non-invasive evaluation of knee joint kinematics with weight bearing. This tool may potentially be used for assessing knee kinematics in patients with knee pathology.

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.

Accuracy and repeatability of a pressure measurement system in the patellofemoral joint

The objective of this study was to assess how accurately and repeatably the Iscan system measures force and pressure in the natural patellofemoral joint. These measurements must be made to test widely held assumptions about the relationships between mechanics, pain and cartilage degeneration. We assessed the system's accuracy by using test rigs in a materials testing machine to apply known forces and force distributions across the sensor. The root mean squared error in measuring resultant force (for five trials at each of seven load levels) was 6.5 +/- 4.4% (mean +/- standard deviation over all trials at all load levels), while the absolute error was -5.5 +/- 5.6%. For force distribution, the root mean squared error (for five trials at each of five force distributions) was 0.86 +/- 0.58%, while the absolute error was -0.22 +/- 1.03%. We assessed the repeatability of the system's measurements of patellofemoral contact force, pressure and force distribution in four cadaver specimens loaded in continuous and static flexion. Variability in measurement (standard deviation expressed as a percentage of the mean) was 9.1% for resultant force measurements and 3.0% for force distribution measurements for static loads, and 7.3% for resultant force and 2.2% for force distribution measurements for continuous flexion. Cementing the sensor to the cartilage lowered readings of resultant force by 31 +/- 32% (mean +/- standard deviation), area by 24 +/- 13% and mean pressure by 9 +/- 34% (relative to the uncemented sensor). Maximum pressure measurement, however, was 24 +/- 43% higher in the cemented sensor than in the uncemented sensor. The results suggest that the sensor measures force distribution more accurately and repeatably than absolute force. A limitation of our work, however, is that the sensor must be cemented to the patellar articular surface to make the force distribution measurements, and our results suggest that this process reduces the accuracy of force, pressure and area measurements. Our results suggest that the Iscan system's pressure measurement accuracy and repeatability are comparable to that of Fuji Prescale film, but its advantages are that it is thinner than most Fuji Prescale film, it measures contact area more accurately and that it makes continuous measurements of force, pressure and area.