Force-controlled dynamic wear testing of total ankle replacements

Biomechanics and Tribology of Total Ankle Replacement

This article discusses the biomechanics and tribology of total ankle replacements considering the influence of implant design and generation on functional outcome, before discussing the interplay between biomechanics and tribology in the clinical success of total ankle replacement. It reflects on what we know and highlights areas for further research, as well as identifying factors to consider in clinical practice.

Is load control necessary to produce physiological AP displacement and axial rotation in wear testing of TAR?

The International Standard Organization, ISO 22622, specifies two options for joint wear simulator evaluation of total ankle replacements (TARs): load-controlled and displacement-controlled. In the present study, the load-controlled testing parameters were applied to cadaveric specimens to quantify and compare the observed sagittal translations and axial rotations to those specified under the displacement-controlled option. Twelve cadaveric specimens were stripped of extraneous tissues, keeping surrounding ankle ligaments. A halo was used to produce plantarflexion and dorsiflexion of the talus through two screws, while a baseplate resisted axial loads. The axial force and torque were applied to the tibia and fibula under force and torque feedback control. An anterior-posterior force was applied to the tibia. Plantarflexion-dorsiflexion were applied using rotation control. To protect the cadaveric specimens, loads were applied at 50% of the specified load profile while plantarflexion-dorsiflexion rotation was applied as specified. There was variation among specimens in magnitudes of anterior-posterior displacement with peaks ranging from 3.3 mm posteriorly to 3.0 mm anteriorly. Likewise, there was variation among specimens in magnitude of axial rotation, with peaks ranging from 11° external rotation to 4.5° internal rotation. However, the mean magnitudes of AP displacement and axial rotation did not exceed those specified by ISO 22622.

A Systematic Review of Unsystematic Total Ankle Replacement Wear Evaluations

BACKGROUND

Numerous studies have reported the use of laboratory multistation joint simulators to successfully predict wear performance and functionality of hip and knee replacements. In contrast, few studies in the peer-reviewed literature have used joint simulation to quantify the wear performance and functionality of ankle replacements. We performed a systematic review of the literature on joint simulator studies that quantified polyethylene wear in total ankle arthroplasty. In addition to the quantified wear results, the load and motion parameters were identified and compared among the studies.

METHODS

A search was performed according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to identify articles reporting total ankle replacement polyethylene wear using joint simulators.

RESULTS

Nine studies that used joint simulators and 1 study that used a computer simulation were found. Although all studies used physiological multidirectional motions (i.e., internal/external rotation, plantar flexion/dorsiflexion, anterior/posterior translation), there was large variability among the studies in the magnitudes of these motions. Among these studies, mean non-cross-linked polyethylene wear ranged from 3.3 ± 0.4 to 25.8 ± 3.1 mm per million cycles. In contrast, mean highly cross-linked polyethylene wear ranged from 2.1 ± 0.3 to 3.3 ± 0.4 mm per million cycles. The wide distribution in wear rates was attributable to the highly inconsistent kinematic parameters and loads applied as well as differences in implant design and materials.

CONCLUSIONS

There is a severe lack of clinically applicable data on wear performance of total ankle replacements in the peer-reviewed literature. No universal set of kinematic load parameters has been established. Furthermore, only 2 of the published studies have validated their findings using independently derived data, such as retrieval analysis. These shortcomings make it difficult to compare findings as a function of design parameters and materials, or to draw clinically relevant conclusions from these simulations. More work is required to enhance the predictive capability of in vitro simulations of total ankle replacements.

CLINICAL RELEVANCE

The results of joint wear simulator studies may not accurately represent in vivo wear of total ankle replacements. Joint simulator studies should establish that they are accurately replicating in vivo wear, thus enabling use of their predictive capabilities for new materials and designs.

Operative Wound Complications Following Total Ankle Arthroplasty

BACKGROUND

Wound complications following total ankle replacement (TAR) potentially lead to devastating consequences. The aim of this study was to compare the operative and demographic differences in patients with and without major wound problems which required operative management. We hypothesized that increased tourniquet and operative time would negatively influence wound healing.

METHODS

We identified a consecutive series of 762 primary TARs performed between December 1999 and April 2014 whose data were prospectively collected. We then identified the subset of patients who required a secondary surgery to treat major wound complications (ie, operative debridement, split-thickness skin grafting, and soft tissue reconstruction). All patients requiring a second surgery had operative wound debridement. We then compared the demographics, operative characteristics, and functional scores to see if any differences existed between patients with and without major wound complications. Clinical outcomes including secondary procedures and implant failure rates were recorded.

RESULTS

Twenty-six patients (3.4%) had a total of 49 operative procedures to treat major wound issues. Eighteen patients had flaps and 14 had split-thickness skin grafts. The median time to operatively treating the wound was 1.9 (range: 0.5-12.5) months after the index TAR. The median follow-up time from the wound procedure was 12.7 (range: 1.2-170.8) months. Compared to the control group, patients with major wounds had a significantly longer mean surgery (214.8 vs 189.3 minutes, P = .041) time and trended toward a longer median tourniquet time (151 vs 141 minutes, P = .060). Patients without wound complications were more likely to have posttraumatic arthritis, whereas those with wound complications were more likely to have primary osteoarthritis ( P = .006). The control group trended toward having a higher mean BMI (29.5 vs 27.2, P = .056). There were 6 failures in the major wound complication cohort (23.1%), including 2 below the knee amputations.

CONCLUSION

Ankle wounds that required operative management had high failure rates and some resulted in devastating outcomes. We did not find any increase in major wound complications in those with various risk factors as identified by other studies. Given our data, we recommend limiting operative time. While correcting hindfoot and midfoot alignment is important for improving patient functionality and survivorship of the implant, thought should be given to staging the TAR if multiple pathologies are to be addressed at the time of surgery to limit operative time.

LEVEL OF EVIDENCE

Level III, retrospective comparative series.