Gapping phenomenon of longitudinal meniscal tears

Objective. To investigate the gapping behaviour of longitudinal meniscal tears. Design. The gap size of longitudinal meniscal tears was measured in porcine knee joints under various joint loading conditions. Background. Many meniscal fixation implants have low pull out forces. However, it is unknown if these forces are less than the forces the implants must resist in vivo. It is also unknown if gapping occurs in longitudinally torn menisci and what joint loads induce gapping. Methods. Longitudinal tears were set in the medial menisci of eight porcine knee joints. To observe the tears an opaque placeholder of the same shape as the original articular surface replaced the medial tibial plateau. The knees were exposed to flexion-extension cycles in a loading and motion simulator under 30 and 200 N axial joint load without external moments, under tibial rotation moments, varus or valgus moments, and combined moments. For each load condition the maximum gap of the tear was registered. Results. Maximum gapping (1.59 mm, SD 0.47) occurred in the 3 cm tear at 200 N axial joint load under the combination of a valgus and external rotation moment. An internal tibial rotation moment produced the smallest gaps in tears of any length and at any axial joint load. Conclusions. The maximum gap size found in this study and the previously published pull out stiffnesses of meniscal fixation implants reveal maximum forces on the implant in vivo of only 8.4 N. This explains why meniscal tears fixed with implants of low pull out strength can heal.