Analysis of joint laxity after total ankle arthroplasty: cadaver study

Preclinical biomechanical testing models for the tibiotalar joint and its replacements: A systematic review

In recent years, total ankle replacements have gained increasing popularity as an alternative to fusion. Preclinical testing of TARs requires reliable in vitro models which, in turn, need thorough knowledge of the kinematics of the tibiotalar joint. Surprisingly few studies have been published to simulate the in vivo kinematics of the tibiotalar joint. Among these studies, there is a wide range of methods and magnitudes of applied loads. The purpose of the present review was to summarize the applied loads, positions that were tested during static simulations, and ranges of motion simulated that have been used in human cadaveric models of the tibiotalar joint. Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, PubMed and Google Scholar were searched for studies pertaining to cadaveric tibiotalar joint kinematics. Our search yielded 12 appropriate articles that were included in the systematic review. While it is well known that loads at the tibiotalar joint are frequently as high as 5 times bodyweight [1], these studies reported applied loads varying from 200N-750N, below average bodyweight. Three studies used dynamic loading of custom apparatuses to drive cadaver limbs along predetermined paths to simulate gait. Conversely, the other nine studies applied static loads (∼300N), performed at discreet points during the stance phase, considerably lower than physiological conditions. The present systematic review calls for an urgent need to establish a consensus for preclinical evaluation of TARs for biomechanical function.

Fixed and mobile-bearing total ankle prostheses: Effect on tibial bone strain

BACKGROUND

Total ankle replacement is associated to a high revision rate. To improve implant survival, the potential advantage of prostheses with fixed bearing compared to mobile bearing is unclear. The objective of this study was to test the hypothesis that fixed and mobile bearing prostheses are associated with different biomechanical quantities typically associated to implant failure.

METHODS

With a validated finite element model, we compared three cases: a prosthesis with a fixed bearing, a prosthesis with a mobile bearing in a centered position, and a prosthesis with mobile bearing in an eccentric position. Both prostheses were obtained from the same manufacturer. They were tested on seven tibias with maximum axial compression force during walking. We tested the hypothesis that there was a difference of bone strain, bone-implant interfacial stress, and bone support between the three cases. We also evaluated, for the three cases, the correlations between bone support, bone strain and bone-implant interfacial stress.

FINDINGS

There were no statistically significant differences between the three cases. Overall, bone support was mainly trabecular, and less effective in the posterior side. Bone strain and bone-implant interfacial stress were strongly correlated to bone support.

INTERPRETATIONS

Even if slight differences are observed between fixed and mobile bearing, it is not enough to put forward the superiority of one of these implants regarding their reaction to axial compression. When associated to the published clinical results, our study provides no argument to warn surgeons against the use of two-components fixed bearing implants.

Motion at the Tibial and Polyethylene Component Interface in a Mobile-Bearing Total Ankle Replacement

BACKGROUND

Normal biomechanics of the ankle joint includes sagittal as well as axial rotation. Current understanding of mobile-bearing motion at the tibial-polyethylene interface in total ankle arthroplasty (TAA) is limited to anterior-posterior (AP) motion of the polyethylene component. The purpose of our study was to define the motion of the polyethylene component in relation to the tibial component in a mobile-bearing TAA in both the sagittal and axial planes in postoperative patients.

METHODS

Patients who were a minimum of 12 months postoperative from a third-generation mobile-bearing TAA were identified. AP images were saved at maximum internal and external rotation, and the lateral images were saved in maximum plantarflexion and dorsiflexion. Sagittal range of motion and AP translation of the polyethylene component were measured from the lateral images. Axial rotation was determined by measuring the relative position of the 2 wires within the polyethylene component on AP internal and external rotation imaging. This relationship was compared to a table developed from fluoroscopic images taken at standardized degrees of axial rotation of a nonimplanted polyethylene with the associated length relationship of the 2 imbedded wires. Sixteen patients were included in this investigation, 9 (56%) were male and average age was 68 (range, 49-80) years. Time from surgery averaged 25 (range, 12-38) months.

RESULTS

Total sagittal range of motion averaged 23±9 (range, 9-33) degrees. Axial motion for total internal and external rotation of the polyethylene component on the tibial component averaged 6±5 (range, 0-18) degrees. AP translation of the polyethylene component relative to the tibial component averaged 1±1 (range, 0-3) mm. There was no relationship between axial rotation or AP translation of the polyethylene component and ankle joint range of motion (P > .05).

CONCLUSION

To our knowledge, this is the first investigation to measure axial and sagittal motion of the polyethylene component at the tibial implant interface in patients following a mobile-bearing TAA. Based on outcome scores and range-of-motion measurements, we believe the patients in this study are a representative cross section of subjects compared to other TAA research results. The results from this investigation indicate the potential for a mobile-bearing TAA to fall within the parameters of normal polyaxial ankle motion. The multiplanar articulation in a mobile-bearing TAA may reduce excessively high peak pressures during the complex dynamic tibial and talar motion, which may have a positive influence on gait pattern, polyethylene wear, and implant longevity.

LEVEL OF EVIDENCE

Level IV, case series.

Force-controlled dynamic wear testing of total ankle replacements

Currently, our knowledge of wear performance in total ankle replacements is limited. The aim of this study is to develop a scenario for force-controlled testing and wear testing of total ankle replacements. A force-controlled wear test was developed: based on cadaver measurements, the passive stabilization (ligaments and soft tissue) of the ankle joint was characterized and a restraint model for ankle stabilization was developed. Kinematics and kinetics acting at the replaced ankle joint were defined based on literature data and gait analysis. Afterwards, force-controlled wear testing was carried out on a mobile, three-component, total ankle replacement design. Wear was assessed gravimetrically and wear particles were analyzed. Wear testing resulted in a mean wear rate of 18.2±1.4mm(3)/10(6) cycles. Wear particles showed a mean size of 0.23μm with an aspect ratio of 1.61±0.96 and a roundness of 0.62±0.14. Wear testing of total ankle replacement shows that a relevant wear mass is generated with wear particles in a biologically relevant size range. The developed wear test provides a basis for future wear testing of total ankle replacements.

Initial instability in total ankle replacement: a cadaveric biomechanical investigation of the STAR and agility prostheses

BACKGROUND

Design improvements have increased the success of total ankle replacement, providing patients with end-stage ankle arthritis a viable alternative to arthrodesis. However, revision rates are higher than those for hip and knee arthroplasty, with the most prevalent cause of failure being aseptic loosening. The objective of this study was to quantify and compare the relative bone-implant motion patterns in two well-known total ankle replacement designs.

METHODS

A custom-designed mechanical simulator applied compressive loads (up to 300 N) and bending moments (3 Nm) to six pairs of human cadaveric ankles implanted with total ankle replacements, inducing a natural range of motion about three orthogonal axes: plantar flexion-dorsiflexion, inversion-eversion, and internal-external rotation. The implants analyzed were the Agility and the STAR (Scandinavian Total Ankle Replacement). The relative bone-implant motions for each implant component were measured with use of an optical motion capture system.

RESULTS

The Agility typically exhibited greater relative motion than the STAR, with significant differences for the tibial component in inversion-eversion (p = 0.037) and for the talar component in internal-external rotation (p = 0.039). The magnitudes of the relative motions were affected by the loading direction and by compression. The motion magnitudes were quite large, with values exceeding 1000 μm for the Agility talar component in plantar flexion-dorsiflexion and in inversion-eversion.

CONCLUSIONS

The greater magnitudes of relative motion in the Agility suggest that primary instability of the implant may contribute to its higher clinically observed aseptic loosening rate. Future total ankle replacement designs will require better fixation to improve outcomes. The results underscore the need to conduct preclinical biomechanical assessments of relative motion patterns in ankle replacements.

CLINICAL RELEVANCE

Stable initial implant fixation will likely improve clinical outcomes of total ankle replacement.

Comparison of sagittal subluxation in two different three-component total ankle replacement systems

BACKGROUND

Malalignment following total ankle arthroplasty (TAA) has been reported in 4% to 45% of patients. However, all reports to date have been related to coronal deformity. This study compared sagittal malalignment between the Mobility and Hintegra total ankle systems and assessed the positional stability of the implant components over time.

METHODS

The study included 50 cases each of total ankle replacement arthroplasty with the Hintegra and Mobility total ankle systems performed between May 2008 and June 2010. The Mobility group included 24 men and 25 women, and the mean age was 60.3 years (range, 50.7-70.0 years). The Hintegra group included 25 men and 25 women, and the mean age was 59.8 years (range, 50.8-68.7 years). The 2 groups did not differ in terms of gender (P = .76) or age (P = .77). Three independent observers with different levels of training evaluated the radiographs and performed the measurements independently. Each observer evaluated the radiographs twice at a 6-week interval to determine the intraobserver reliability, and the anteroposterior offset ratio was evaluated.

RESULTS

The anteroposterior offset ratio intra- and interobserver reliabilities all showed good or excellent levels of agreement in the Hintegra total ankle system and the Mobility total ankle system. With respect to the stability of sagittal translation of the talus, the Mobility system (0.08 ± 0.07 immediately, 0.0 ± 0.07 at 6 weeks postoperatively, and 0.01 ± 0.07 at 1 year postoperatively) was better than the Hintegra system (0.20 ± 0.08 immediately, 0.18 ± 0.11 at 6 weeks postoperatively, and 0.15 ± 0.10 at 1 year postoperatively) (P < .0001).

CONCLUSIONS

The Mobility system had less sagittal malalignment of the talus than the Hintegra system. Consequently, when treating ankles in patients with osteoarthritis using the Hintegra system, one must pay careful attention to sagittal malalignment during surgery.

LEVEL OF EVIDENCE

Level III, retrospective comparative series.

The role of ankle ligaments and articular geometry in stabilizing the ankle

BACKGROUND

Ankle joint stability is a function of multiple factors, but it is unclear to what extent extrinsic factors such as ligaments and intrinsic elements such as geometry of the articular surfaces play a role. The purposes of this study were to determine the contribution of the ligaments and the articular geometry to ankle stability and to determine the effects of ankle position and simulated physiological loading upon ankle stability.

METHODS

Sixteen cadaveric lower extremities were studied in unloaded and with axial load equivalent to body weight. Anterior-posterior, medial-lateral translation and internal-external rotation tests were performed in neutral, dorsiflexion and plantarflexion ankle positions. Intact ankle stability was measured; ankle ligaments were serially sectioned and retested.

FINDINGS

For unloaded condition, the lateral ligament accounted for 70% to 80% of anterior stability and the deltoid ligaments for 50% to 80% of posterior stability. Both ligaments contributed 50% to 80% to rotational stability; however, the ligaments did not provide the primary restraints to medial-lateral stability. For loaded ankle condition, articular geometry contributed 100% to translational and 60% to rotational stability. The ankle was less stable in plantarflexion and more stable in dorsiflexion.

INTERPRETATION

The contribution of extrinsic and intrinsic elements to ankle stability is dependent upon the load and direction of force applied. This study underscores the importance of restoring soft tissues about the ankle to the anatomic condition during reconstruction operations for instability, trauma and arthritis.

Position of the prosthesis components in total ankle replacement and the effect on motion at the replaced joint

PURPOSE

In some cases of total ankle replacement, perfect alignment of the prosthetic components is not achieved. This study analyses the extent to which component positioning is critical for the final range of motion.

METHODS

Fourteen patients undergoing total ankle replacement were assessed preoperatively and postoperatively at seven and 13 months follow-up. X-ray pictures of the ankle were taken in static double leg stance, i.e. at neutral joint position, and in maximum plantarflexion and dorsiflexion. Measurements were obtained by a specially devised computer program based on anatomical reference points digitised on the radiograms. These allowed calculation of the position and orientation of the components in the sagittal and coronal planes, together with the joint range of motion.

RESULTS

The mean range of motion was about 34 degrees at the first follow-up and maintained at the second. Tibial and talar components were more anterior than the mid-tibial shaft in 11 and nine patients, respectively. Mean inclination was about four degrees posterior for the tibial component and nearly one degree anterior for the talar component. A significantly larger range of motion was found in ankles both with the talar component located and inclined more anteriorly than the tibial.

CONCLUSIONS

Correlation, though weak, was found between motion at the replaced ankle and possible residual subluxation and inclination of the components. However, a satisfactory range of motion was also achieved in those patients where recommended locations for the components could not be reached because of the size of the original joint deformity.

The relation between geometry and function of the ankle joint complex: a biomechanical review

This review deals with the relation between the anatomy and function of the ankle joint complex. The questions addressed are how high do the forces in the ankle joint get, where can the joints go (range of motion) and where do they go during walking and running. Finally the role of the ligaments and the articular surfaces is discussed, i.e. how does it happen. The magnitude of the loads on the ankle joint complex are primarily determined by muscle activity and can be as high as four times the body weight during walking. For the maximal range of motion, plantar and dorsiflexion occurs in the talocrural joint and marginally at the subtalar joint. In-eversion takes place at both levels. The functional range of motion is well within the limits of the maximal range of motion. The ligaments do not contribute to the forces for the functional range of motion but determine the maximal range of motion together with the articular surfaces. The geometry of the articular surfaces primarily determines the kinematics. Clinical studies must include these anatomical aspects to better understand the mechanism of injury, recovery, and interventions. Models can elucidate the mechanism by which the anatomy relates to the function. The relation between the anatomy and mechanical properties of the joint structures and joint function should be considered for diagnosis and treatment of ankle joint pathology.