Pre-clinical assessment of total knee replacement anterior-posterior constraint.

Pre-clinical, bench-top assessment of Total Knee Replacements (TKR) can provide information about the inherent constraint provided by a TKR, which does not depend on the condition of the patient undergoing the arthroplasty. However little guidance is given by the ASTM standard on test configurations such as medial-lateral (M:L) loading distribution, flexion angle or restriction of secondary motions. Using a purpose built rig for a materials testing machine, four TKRs currently in widespread clinical use, including medial-pivot and symmetrical condyle types, were tested for anterior-posterior translational constraint. Compressive joint loads from 710 to 2000 N, and a range of medial-lateral (M:L) load distributions, from 70:30% to 30:70% M:L, were applied at different flexion angles with secondary motions unconstrained. It was found that TKA constraint was significantly less at 60 and 90° flexion than at 0°, whilst increasing the compressive joint load increased the force required to translate the tibia to limits of AP constraint at all flexion angles tested. Additionally when M:L load distribution was shifted medially, a coupled internal rotation was observed with anterior translation and external rotation with posterior translation. This paper includes some recommendations for future development of pre-clinical testing methods.

Evaluation of different shape-memory staple configurations against crossed screws for first metatarsophalangeal joint arthrodesis: A biomechanical study.

BACKGROUND: The first metatarsophalangeal joint may be fused in order to treat arthritis or instability. The use of shape-memory staples for fixation is well recognised, but little work has been done into the optimal configuration of staples. METHODS: The structural behaviour of first metatarsophalangeal joint (MTPJ) arthrodeses using shape-memory staples or crossed screws was studied using cadaveric porcine joints. Five fixation configurations were tested: single vertical or horizontal staple, paired staples in dorsal-medial configuration (0-90° to the sagittal plane), paired staples in oblique orthogonal configuration (45-135°); or two crossed screws. Specimens were loaded in cyclical dorsiflexion for 1000 cycles. Plantar gapping and shearing were measured. Specimens were then loaded to failure. RESULTS: Cyclic testing caused more shear in the 45-135° staples than the crossed screws (1.0mm±0.5mm compared to 0.14mm±0.4mm, p<0.01). No significant difference was found in plantar gap formation. Single vertical and horizontal single staples failed at 15N and 19. CONCLUSIONS: N, respectively. Paired 0-90° staples failed at 43N±9N, significantly lower than the 45-135° staples (141N±25N; p<0.001) and crossed screws (180N±67N; p<0.001). There was no significant difference between the 45-135° staples and crossed screws. Screws failed by sudden cortical fracture; staples displayed gradual pull-out and shearing. First MTPJ arthrodeses fixed with single staples are not recommended. Arthrodeses fixed with staples at 0-90° to the sagittal plane were significantly less strong than two crossed screws. However, positioning oblique staples at 45-135° significantly improved stability, creating a construct as strong as, crossed screws. None of the constructs was strong enough for immediate weight bearing.

The anterolateral ligament: Anatomy, length changes and association with the Segond fracture.

There have been differing descriptions of the anterolateral structures of the knee, and not all have been named or described clearly. The aim of this study was to provide a clear anatomical interpretation of these structures. We dissected 40 fresh-frozen cadaveric knees to view the relevant anatomy and identified a consistent structure in 33 knees (83%); we termed this the anterolateral ligament of the knee. This structure passes antero-distally from an attachment proximal and posterior to the lateral femoral epicondyle to the margin of the lateral tibial plateau, approximately midway between Gerdy's tubercle and the head of the fibula. The ligament is superficial to the lateral (fibular) collateral ligament proximally, from which it is distinct, and separate from the capsule of the knee. In the eight knees in which it was measured, we observed that the ligament was isometric from 0° to 60° of flexion of the knee, then slackened when the knee flexed further to 90° and was lengthened by imposing tibial internal rotation.