Ligament and Tendon Enthesis: Anatomy and Mechanics - Structural Interfaces and Attachments in Biology - page 71

Fig. 4.4 A photomicrograph
showing the distal (tibial)
insertion of the MCL from a
skeletally immature rabbit.
Note that the osteoclasts (OC)
and osteoblast (OB) and the
oblique deeper fibers'
insertion to bone have not yet
been well established
(reproduced, with permission,
from [ 14 ])
found that the insertion site on the tibial side is affected by its proximity to the
growth plate where (1) osteoclastic activity in this region weakens the subperiosteal
attachment and (2) part of the ligament insertion is at the area of the metaphysis.
The examination of the tibial insertions of skeletally immature rabbit specimens
revealed several differences when compared to mature samples. The metaphysis of
the proximal tibia in the skeletally immature rabbit was composed of bars
containing calcified cartilage, and the primary spongy bone was undergoing reor-
ganization as exemplified by the increased presence of osteoclasts and osteoblasts
(Fig. 4.4 ). The delayed maturation of the MCL-tibia junction may be due to
this increased complexity compared to that of the MCL substance. Therefore, this
junction was an area of weakness in the FMTC and the skeletally immature
specimens failed by tibial avulsion. This finding was well-illustrated by histological
evaluation of the components of the FMTC, which showed that the weakest link in
skeletally immature rabbits was the tibial insertion, a result of the maturation
process that included bone remodeling activities in the subperiosteal region of
this attachment. Thus, some of the deep oblique fibers had not yet attached to
bone and the ligament attachment became more dependent on the periosteal
component. In contrast, the femoral insertion had a direct penetration of fibers
into the metaphyseal cortex. There were no histological differences noted at the
femoral insertion of the MCL as a function of skeletal maturity.
4.3.1.3 Skeletally Mature
As the insertion site changes structurally and compositionally with skeletal matu-
ration, there are also significant changes in the structural properties of the
femur-MCL-tibia complex during maturation of the rat and rabbit. At this indirect
insertion, skeletal maturity is characterized by the deep fibers anchoring solidly into
the bone. Tipton and associates [ 13 ] and our research center [ 14 , 15 ] found that in
the rabbit, the strength of the FMTC did not plateau until closure of the epiphyses at
about 7 months of age, and tensile failure of skeletally immature animals occurred
by tibial avulsion in all cases [ 14 ]. In contrast, older rabbit specimens exhibited