Anatomical variation of the distal femur - a CT data analysis of 24,042 knees

Based on a large quantity of CT data, variations in distal femoral geometry was examined and evaluated for TKA. A retrospective study was performed on 24,042 data sets generated during the process of designing individual knee implants. Following parameters were recorded for the distal femur: Femoral absolute anterior-posterior (AP) and medial-lateral (ML) extent, lateral and medial condyle and trochlea size, distal condylar offset (DCO) between lateral and medial condyle, and the difference between medial and lateral posterior condylar offset (PCO) measured in AP direction. Variable patient geometry was found with analysis of the AP and ML extent. Approximately one-third of the patients would experience size conflicts of +/- 3 mm with standard arthroplasty systems. 62% of the knees had a DCO> 1 mm. 83% of the distal femur had a mediolateral difference in PCO> 2 mm, which corresponds to about 3° external rotation and does not correlate with the femoral size. There is a distinct variability of femoral AP and ML extent as well as offsets / asymmetries. Medial and lateral PCOs are different and do not correlate with femoral size. This first results in mismatches between size of implant and individual knee anatomy and secondly in possible softtissue release and different femoral external rotations to adapt systems with fixed distal geometry to the individual situation.

Surgical technique

The trays containing the tibial instruments, templates and trial components and those used with all sizes of femur are shown in Figure 7.1. A numbered list of illustrations of all the instruments, trial components and templates can be found at the end of this chapter. When a component or instrument is mentioned first in the following text, its number will be printed with square brackets. The five sizes of femoral component have different spherical radii of curvature. For each femoral size, there is a matching set of men iscal bearings in seven thicknesses, from 3 mm to 9 mm. There is a separate tray of instruments for each femoral size. The trays, one of which is shown in Figure 7.2, contain colour coded instruments and trial components specifically for use with one size of femoral component. They must not be mixed up so it is safer just to open one size.

Changes in mineralization and biomechanics of tibial metaphyses in splinted rats

The effect of 3 wk of splintage of a single hindlimb on the midarea and mineral content of both tibial metaphyses was assessed immediately after splint removal and after 1 mo of mobilization in 12-wk-old Sprague-Dawley rats. Immobilization reduced tibial metaphyseal bone mineral density (BMD) in immobilized limbs compared with “free” limbs of splinted animals and with controls. These changes persisted and were accentuated by relatively greater increases in tibial metaphyseal BMDs of unsplinted (control) animals after 7 wk. Immediately after splintage, tibial metaphyseal areas and total mineral contents of both hindlimbs of splinted animals were reduced compared with those of unsplinted animals. However, the relationship between mineralization and area differed between the free and immobilized limbs of splinted animals. The breaking strain and the breaking energy of immobilized and free femurs of splinted animals were impaired 4 wk after the removal of the splint. This impairment was correlated with an effect of splintage on femoral size with some additional local effect from immobilization. Thus osteoporotic changes consequent on immobilization include both local effects on mineralization and general effects on growth, which may separately influence the elastic properties of bone.