The effect of knee prosthesis design on tibiofemoral biomechanics during extension tasks following total knee arthroplasty.

BACKGROUND: Determine whether the tibiofemoral motion and electromyographic activity of the knee differs in patients with a medial pivot implant, compared to those with cruciate-retaining and posterior-stabilised designs, during knee extension after Total Knee Arthroplasty (TKA). METHODS: An observational study was conducted on a cohort of patients that had undergone TKA for a minimum of 12 months prior. Three matched groups (n = 18) were categorised based on implant type: medial-pivot (MP), posterior-stabilised (PS) and cruciate-retaining (CR). Kinematics, with motion analysis (Vicon, USA) and surface electromyography (Delsys, USA) were assessed during step-ascent and walking tasks. RESULTS: All groups displayed a similar amount of knee extension in both tasks. They also paradoxically produced an average mean internal rotation movement during knee extension in both the step-ascent and walking tasks. The only significant difference was found in the step-ascent task, in which the MP group produced a larger absolute amount of rotation than the CR implant group (P = 0.007), but neither group differed from the PS implant group. The groups did not differ in rotation during the walking task (P > 0.05). The MP group displayed significantly (P < 0.01) greater knee extensor activation during the step-ascent than the PS group. CONCLUSION: The MP design was only significantly different to another implant design for the step-ascent task. Patients with either knee implant types were not strictly limited to producing the traditional "screw-home" mechanism, defined by external rotation during extension. Furthermore, comparison with the non-implant contralateral limb suggested that rotation is not necessarily dictated by implant design.

Mechanical properties of gamma irradiated morselized bone during compaction.

This study examined the effects of gamma irradiation on the compressive properties of morselized cancellous bone from human femoral heads. Twelve bone samples, mean age of 68 years (range 92-39), were divided into 3 groups (N=12) of varying irradiation level (0, 15 and 25 kGy). Each specimen was compacted in a controlled fashion in steps of 0.5 mm at 0.5 mm/min (up 12 mm). The load and stiffness increased with compaction, but this relationship was not linear. There was no statistical significant difference in the compacting load or stiffness between groups (p>0.05) until the last 1 mm of compaction, where the 25 kGy group were significantly stiffer compared to controls but not different to the 15 kGy group. This may be due to decreased interlocking of bone particles caused by higher irradiation levels resulting in a stiffer graft.