Biomechanical comparison of stability of tibiotalocalcaneal arthrodesis with two different intramedullary retrograde nails.

BACKGROUND: The aim of the study was to compare the initial construct stability of two retrograde intramedullary nail systems for tibiotalocalcaneal arthrodesis (TTCF) (A3, Small Bone Innovations; HAN, Synthes) in a biomechanical cadaver study. METHODS: Nine pairs of human cadaver bones were instrumented with two different retrograde nail systems. One tibia from each pair was randomized to either rod. The bone mineral density was determined via tomography to ensure the characteristics in each pair of tibiae were similar. All tests were performed in load-control. Displacements and forces were acquired by the sensors of the machine at a rate of 64Hz. Specimens were tested in a stepwise progression starting with six times ±125N with a frequency of 1Hz for 250cycles each step was performed (1500cycles). The maximum load was then increased to ±250N for another 14 steps or until specimen failure occurred (up to 3500cycles). RESULTS: Average bone mineral density was 67.4mgHA/ccm and did not differ significantly between groups (t-test, p=.28). Under cyclic loading, the range of motion (dorsiflexion/plantarflexion) at 250N was significantly lower for the HAN-group with 7.2±2.3mm compared to the A3-group with 11.8±2.9mm (t-test, p<0.01). Failure was registered for the HAN after 4571±1134cycles and after 2344±1195cycles for the A3 (t-test, p=.031). Bone mineral density significantly correlated with the number of cycles to failure in both groups (Spearman-Rho, r>.69, p<0.01). CONCLUSIONS: The high specimen age and low bone density simulates an osteoporotic bone situation. The HAN with only lateral distal bend but two calcaneal locking screws showed higher stability (higher number of cycles to failure and lower motion such as dorsiflexion/plantarflexion during cyclic loading) than the A3 with additional distal dorsal bend but only one calcaneal locking screw. Both constructs showed sufficient stability compared with earlier data from a similar test model. CLINICAL RELEVANCE: The data suggest that both implants allow for sufficient primary stability for TTCF in osteoporotic and consequently also in non-osteoporotic bone. LEVEL OF EVIDENCE: Not applicable, experimental basic science study.

Hindfoot joint pressure in supination sprains.

BACKGROUND: Hindfoot trauma including ankle and subtalar sprains may be followed by osteochondral lesions and persisting pain originating from posttraumatic arthritis. HYPOTHESIS: "Acute severe" supination sprains and "mild supination sprains with increasing ligamentous incompetence" of the hindfoot lead to medial ankle and subtalar stress concentrations that may contribute to osteochondral lesions (OCLs) and subsequent arthritis. STUDY DESIGN: Controlled laboratory study. METHODS: "Acute severe" sprains and "mild sprains with increasing ligamentous incompetence" (consecutive anterior talofibular ligament [ATFL], calcaneofibular ligament [CFL], and lateral talocalcaneal ligament [LTCL] transections) were simulated in human lower leg specimens (7 different specimens for each condition). The effect on the migration of the center of force (COF) and on the tibiotalar and subtalar pressures at 700-N (acute severe) and 150-N (mild sprain with increasing ligamentous incompetence) axial static and dynamic loads, respectively, was recorded using pressure sensors. RESULTS: In the "acute severe" sprain, the peak pressure increase reached the level of significance in the ankle (P = .042) and in the subtalar medial facet (P = .046). The ankle COF migrated significantly toward the medial (P = .001) and posterior (P = .023) directions. In the "mild sprain with increasing ligamentous incompetence" condition, the ankle (P = .018) and subtalar (medial facet, P = .022) peak pressure increased significantly with intact ligaments and with all ligaments cut. The ankle COF migrated significantly toward the medial direction when the ATFL and CFL or when all 3 ligaments were severed. The anteroposterior ankle COF migration was anterior when all ligaments were intact or when only the ATFL was severed but posterior when the CFL or when the CFL and the LTCL were severed in addition. CONCLUSION: Next to chronic inhomogeneous load distribution in the unstable hindfoot and shear stress during sprains, intra-articular pressure elevation (impact) in the ankle and subtalar joint during hindfoot supination sprains with intact ligaments or incompetent ligaments likely contributes to OCLs of the medial talar dome and the medial subtalar facet. CLINICAL RELEVANCE: Intra-articular hindfoot pressure elevation (impact) in "acute severe" and in "mild hindfoot supination sprains with increasing ligamentous incompetence" is substantial for the development of OCLs at the medial midtalar dome and the medial facet of the subtalar joint.

Ankle joint pressure changes in a pes cavovarus model after lateralizing calcaneal osteotomies.

BACKGROUND: Tendon transfers and calcaneal osteotomies are commonly used to treat symptoms related to medial ankle arthrosis in fixed pes cavovarus. However, the relative effect of these osteotomies in terms of lateralizing the ground contact point of the hindfoot and redistributing ankle joint contact stresses are unknown. MATERIALS AND METHODS: Pes cavovarus with fixed hindfoot varus was simulated in eight cadaver specimens. The effect of three types of calcaneal osteotomies on the migration of the center of force and tibiotalar peak pressure at 300 N axial static load (half-body weight) were recorded using pressure sensors. RESULTS: A significant lateral shift of the center of force was observed: 4.9 mm for the laterally closing Z-shaped osteotomy with additional lateralization of the tuberosity, 3.4 mm for the lateral sliding osteotomy of the calcaneal tuberosity, and 2.7 mm for the laterally closing Z-shaped osteotomy (all p < 0.001). A significant peak pressure reduction was recorded: -0.53 MPa for the Z-shaped osteotomy with lateralization, -0.58 MPa for the lateral sliding osteotomy of the calcaneal tuberosity, and -0.41 MPa for the Z-shaped osteotomy (all p < 0.01). CONCLUSION: This cadaver study supports the hypothesis that lateralizing calcaneal osteotomies substantially help to normalize ankle contact stresses in pes cavovarus.