The fibular meniscus of the kangaroo as an adaptation against external tibial rotation during saltatorial locomotion

Abstract

The kangaroo knee is, as in other species, a complex diarthrodial joint dependent on interacting osseous, cartilaginous and ligamentous components for its stability. While principal load bearing occurs through the femorotibial articulation, additional lateral articulations involving the fibula and lateral fabella also contribute to the functional arrangement. Several fibrocartilage and ligamentous structures in this joint remain unexplained or have been misunderstood in previous studies. In this study, we review the existing literature on the structure of the kangaroo 'knee' before providing a new description of the gross anatomical and histological structures. In particular, we present strong evidence that the previously described 'femorofibular disc' is best described as a fibular meniscus on the basis of its gross and histological anatomy. Further, we found it to be joined by a distinct tendinous tract connecting one belly of the m. gastrocnemius with the lateral meniscus, via a hyaline cartilage cornu of the enlarged lateral fabella. The complex of ligaments connecting the fibular meniscus to the surrounding connective tissues and muscles appears to provide a strong resistance to external rotation of the tibia, via the restriction of independent movement of the proximal fibula. We suggest this may be an adaptation to resist the rotational torque applied across the joint during bipedal saltatory locomotion in kangaroos.

Keywords: cyamella; fabella; femorofibular joint; menisci; rotational torque.

Figure 1

Dissection of the knee of adult Macropus fuliginosus: (a) lateral aspect; (b) lateral aspect flexed position and lateral collateral ligament transected; and (c) caudal aspect. ccl, caudal cruciate ligament; cml, caudal meniscal ligament; cmtl, caudal meniscotibial ligament; edl, m. extensor digitorum longus; fab, lateral fabella; fbm, fibular meniscus; fmfl, fibular meniscofemoral ligament; gce, m. gastrocnemius externus; lcl, lateral collateral ligament; lfc, lateral femoral condyle; lfe, lateral femoral epicondyle; lm, lateral (femorotibial) meniscus; lmfl, lateral meniscofemoral ligament; mfc, medial femoral condyle; mfl, meniscofabellar ligament; mtc, medial tibial condyle; mm, medial (femorotibial) meniscus; pop, m. popliteus; ptf, proximal tibiofibular joint. Scale bar: 20 mm.

Figure 2

Lateral meniscus complex and fabella of adult Macropus fuliginosus, proximal view. clb, caudolateral band of cranial cruciate ligament; cmb, craniomedial band of cranial cruciate ligament; fab, lateral fabella; fbm, fibular meniscus; gce, m. gastrocnemius externus; ice, intercondylar eminence; ifp, infrapatellar fat pad; iml, intermeniscal ligament; lm, lateral (femorotibial) meniscus; ltp, lateral tibial pad; mcl, medial collateral ligament (transected); mfl, menicofabellar ligament; mm, medial (femorotibial) meniscus; mtp, medial tibial pad; pop, m. popliteus (transected). Note the smooth articular facet on the medial aspect of the fabellar cornu, and the deep fissure (*) separating the stacked dorsal (intermeniscal) and ventral (meniscofabellar) ligaments cranial to the fibrocartilage body of the fibular meniscus (fbm). Scale bar: 20 mm.

Figure 3

Composite of histological sections of adult female kangaroo, low‐power plan and details (A–E) at 100 ×. Plan and detail (E), Martius Scarlet Blue; details (A–D), Sirius Red under crossed polarized light. The meniscofabellar (*) and intermeniscal (×) ligament tracts are marked. (a) Detail of lateral meniscus (lm); (B) detail of meniscofabellar ligament; (C) detail of fibular meniscus (fbm, top) and underlying meniscofabellar ligament (bottom); (D) dense fibrocartilage at inflection point of meniscofabellar ligament tract; (E) junction of hyaline cartilage fabellar cornu (fc) and meniscofabellar ligament. Scale bar: 10 mm.

Figure 4

Histological section of kangaroo pouch young (141 g, ~90 days). (a) Martius Scarlet Blue, 40 ×; (b) Sirius Red under crossed polarized light, 40 ×; (c) detail of fibular meniscus, 100 ×. fem, femur; fbm, fibular meniscus; fib, fibular head. Note continuity of the ligamentous tract between the fabellar cornu and lateral meniscus (meniscofabellar ligament, marked with *), as distinct from the intermeniscal ligament dorsal (proximal) to it (marked with ×). Scale bar: 1 mm.

Figure 5

Histological section of kangaroo pouch young (34 g, ~50 days), Martius Scarlet Blue, 40 ×; fem, femur; fbm, fibular meniscus (dense mass of fibrocartilage; no hyaline stage); fc, fabellar cornu (note the entirely hyaline nature of the cornu); fib, fibula; tib, tibia. Note continuity of the ligamentous tract between the fabellar cornu and lateral meniscus (meniscofabellar ligament, marked with *), as distinct from the intermeniscal ligament dorsal (proximal) to it (marked with ×).