Subtrochanteric Frx: Anatomy and Factors in Bone Healing

Anatomy

• axis of femoral neck forms angle of approx 135 deg w/ femoral shaft, and is anteverted from the frontal plane by variable amount (- 4 to 36 deg, average 10 to 20 deg in Caucasians);
• distinct cortex surrounds the cancellous bone of the neck, the center of which becomes progressively empty with age (Ward's triangle);
  • center of femoral head is most secure site for proximal fixation;
• at femoral neck, there is transition from metaphysis to epiphysis, formed of cancellous bone that is most dense peripherally, esp medially & laterally where primary compression and primary tension trabecular streams are respectively situated;
• calcar femorale is a condensed IM bony thickening that extends from posterior neck thru the posteromedial portion of intertrochanteric region, just anterior to lesser trochanter, & ends in posteromedial cortex of the proximal shaft;
  • it thus provides sig support to counteract large compressive stresses borne by this region;
  • its displacement, w/ large lesser troc frx fragment, renders unstable a fracture fragment in this region;
• bending forces cause the medial cortex to be loaded in compression & lateral cortex to be in tension;
• compressive forces are much greater than the tension forces, and therefore, it is mandatory to restore medial cortical stability;
  • as little as 2 mm of separation of the medial cortex will lead to medial collapse and lateral plate bending;

Factors in Bone Healing

• major obstacles: non union and malunion;
• factors responsible for slower rate of union or nonunion include:
  • large quantity of cortical bone, which is often comminuted in these frx;
  • cortical bone vascularity and fractures srufaces available for healing are less than in cancellous bone surfaces in intertrochanteric frx;
  • biomechanical stresses across frx may result in hardware failure;
  • stress on nail plate acting as tension band under any condition other than perfect reduction is highly sensitive to loading;
  • failure of nail plate devices in such situations is thus almost certain if early wt bearing is allowed