Neglected, semimembranosus osteochondral avulsion fracture of the posteromedial tibial plateau

Abstract

Semimembranosus avulsion fracture is infrequently reported and is easy to miss on plain radiographs; the mechanism of injury is highly controversial. Initial reports linked it to anterior cruciate ligament and medial meniscal tears. We report an osteochondral semimembranosus avulsion fracture of the posteromedial tibial plateau with associated posterior cruciate ligament rupture. Also described is a novel surgical fixation technique for such osteochondral fractures where the surgical exposure is limited due to the obliquity of the fracture line resulting in a greater involvement of the articular cartilage than the small bony component. The fixation technique described may be used for osteochondral fractures where the application of a conventional compression screw may not be feasible.

Keywords: Neglected; Nonunion; Osteochondral fracture; Posteromedial tibial plateau; Semimembranosus avulsion fracture.

Fig. 1

Plain radiographs (frontal and lateral views) demonstrates a fracture of the posteromedial tibial plateau (on presentation at our outpatient clinic nine months post injury).

Fig. 2

CT images delineating the displaced fragment. Note that the displaced fragment has rotated 90° in the sagittal plane.

Fig. 3

Magnetic resonance images showing intact ACL, ruptured PCL, the attachment of semimembranosus tendon to the osteochondral fragment and the extent of the chondral injury. Note that the size of the bony fragment is disproportionately smaller than the chondral component.

Fig. 4

Intra-operative images showing re-creation of crater walls and the subsequent fixation with screw-washers and screw-posts.

Fig. 5

Immediate post-operative lateral and frontal radiological images showing accurately reduced posteromedial osteochondral fragment.

Fig. 6

Plain radiographs (frontal and lateral views) at two years follow-up showing bony union.

Fig. 7

Illustration showing the principle of fixation used in this case. Arrows a, b, c demonstrate the inherent stability afforded by the intact crater walls and the femoral condyles from above. Arrow d demonstrates the anteriorly-directed buttressing action of the screw head placed posterior to the avulsed fragment.