24 research outputs found
Fixed-bearing Medial Unicompartmental Knee Arthroplasty Restores Neither the Medial Pivoting Behavior Nor the Ligament Forces of the Intact Knee in Passive Flexion
Medial unicompartmental knee arthroplasty (UKA) is an accepted treatment for isolated medial osteoarthritis. However, using an improper thickness for the tibial component may contribute to early failure of the prosthesis or disease progression in the unreplaced lateral compartment. Little is known of the effect of insert thickness on both knee kinematics and ligament forces. Therefore, a computational model of the tibiofemoral joint was used to determine how non-conforming, fixed bearing medial UKA affects tibiofemoral kinematics and tension in the medial collateral ligament (MCL) and the anterior cruciate ligament (ACL) during passive knee flexion. Fixed bearing medial UKA could not maintain the medial pivoting that occurred in the intact knee from 0° to 30° of passive flexion. Abnormal anterior-posterior (AP) translations of the femoral condyles relative to the tibia delayed coupled internal tibial rotation, which occurred in the intact knee from 0° to 30° flexion, but occurred from 30° to 90° flexion following UKA. Increasing or decreasing tibial insert thickness following medial UKA also failed to restore the medial pivoting behavior of the intact knee despite modulating MCL and ACL forces. Reduced AP constraint in non-conforming medial UKA relative to the intact knee leads to abnormal condylar translations regardless of insert thickness even with intact cruciate and collateral ligaments. This finding suggests that the conformity of the medial compartment as driven by the medial meniscus and articular morphology plays an important role in controlling AP condylar translations in the intact tibiofemoral joint during passive flexion
Relative Head Size Increase Using an Anatomic Dual Mobility Hip Prosthesis Compared to Traditional Hip Arthroplasty: Impact on Hip Stability
Smaller head sizes and head/cup ratios make cups smaller than 50mm and larger than 58mm, more prone to dislocation. Using computer modeling, we compared average head sizes and posterior horizontal dislocation distance (PHDD) in two 78-patient matched cohorts. Cup sizes were small (≤50mm) or large (≥58mm). The control cohort had conventional fixed bearing prostheses, while the experimental cohort had anatomical dual mobility (ADM) hip prostheses. ADM cups have larger average head sizes and PHDD than traditional fixed bearing prostheses by 11.5mm and 80% for cups ≤50mm, and 16.3mm and 90% for cups ≥58mm. Larger head sizes and increased head/cup ratio may allow the ADM prosthesis to reduce the incidence of dislocation
The effect of constraint on post damage in total knee arthroplasty: posterior stabilized vs posterior stabilized constrained inserts
Posterior stabilized constrained (PSC) inserts are intended to provide greater varus-valgus and rotational constraint than conventional PS inserts. We determined whether the added constraint resulted in more damage to the post in PSC compared to PS inserts. Retrieved PSC inserts were matched to retrieved PS inserts from the same manufacturer according to patient age, body mass index, and length of implantation. Surface damage was visually assessed, and 3-D surface deviation from pristine was measured. Damage scores for the PSC posts were significantly greater than those of the PS posts. Surface deviation was significantly greater in the posterior and medial post regions of the PSC inserts. Based on short-term follow-up, our results suggest that added constraint is accompanied by greater polyethylene surface damage
High Stress Conditions Do Not Increase Wear of Thin Highly Crosslinked UHMWPE
Introduction of highly crosslinked polyethylene has increased interest in large femoral heads, because thin acetabular liners can be used while maintaining low wear rates and larger heads decrease the incidence of instability. However, crosslinking and subsequent thermal treatments can cause decreased mechanical properties that might obviate the reduced wear under extreme conditions. To examine whether increased contact pressures would adversely affect wear in thin liners, we tested thin and thick highly crosslinked liners (3.8 mm thickness/44-mm head and 7.9 mm thickness/36-mm head, respectively) to 5 million cycles on a hip simulator under near impingement conditions. Conventional polyethylene liners (7.9 mm thickness/36-mm head) served as controls. Large femoral heads with highly crosslinked polyethylene liners as thin as 3.8 mm in thickness do not wear at a higher rate than a thicker liner of the same material, even when subjected to large contact pressures such as occur under near-impingement conditions. Crosslinked polyethylene may allow for liners that are thinner than has been traditionally accepted. This conclusion, however, is based solely on wear test results with idealized cup position, no intentional edge loading, no head subluxation, and no artificial aging. Continued monitoring will be necessary to elucidate the clinical efficacy of these devices
Assessment of Damage on a Dual Mobility Acetabular System
Background: Dual mobility designs were introduced to increase stability and reduce the risk of dislocation, both being common reasons for surgical revision after total hip arthroplasty. The in vivo behavior of dual mobility constructs remains unclear, and to our knowledge, no data have been published describing in vivo surface damage to the polyethylene bearing surfaces.
Methods: We used surface damage assessed on the inner and outer polyethylene bearing surfaces in 33 short-term retrieved dual mobility liners as evidence of relative motion at the 2 bearings. A lever out test was performed to determine the force required for dislocation of the cobalt-chromium femoral head from the polyethylene liner.
Results: Both bearings showed damage; however, the inner polyethylene bearings had higher damage scores, lower prevalence of remaining machining marks, and higher incidence of concentric wear, all consistent with more motion at the inner polyethylene bearing. The inner polyethylene bearings also had a higher occurrence of embedded titanium debris. The damage sustained in vivo was insufficient to lead to intraprosthetic dislocation in any of the retrieved components. Lever out tests of 12 retrievals had a mean dislocation load of 261 +/- 52 N, which was unrelated to the length of implantation.
Conclusion: Our short-term retrieval data of 33 highly cross-linked polyethylene dual mobility components suggest that although motion occurs at both bearing articulations, the motion of the femoral head against the inner polyethylene bearing dominates. Although damage was not severe enough to lead to intraprosthetic dislocation, failure may occur long term and should be assessed in future studies. (C) 2016 Elsevier Inc. All rights reserved