Morphology of the Greater Trochanter: An Assessment of Anatomic Variation and Canal Overhang

Abstract

Background: Varus malposition is a risk of early failure in total hip arthroplasty. The degree to which the tip of the greater trochanter (GT) overhangs the canal can increase this risk. Although we know proximal femoral anatomy is variable, no study has addressed variations in medial overhang of the GT on plain radiographs.

Methods: All low anteroposterior pelvis radiographs more than 1 year were reviewed 3 times by 2 orthopaedic surgeons and one radiologist. The canal width (CW) was measured 10 cm below the lesser trochanter. Canal overhang (CO) was defined by the distance between the lateral medullary canal and a parallel line beginning at the most medial aspect of the GT. The overhang index (OI) is defined as the percentage of the canal overhung by the GT.

Results: The mean CW was 13.5 mm, mean CO 16.4 mm, and mean OI 1.22. Hips were then classified as the following: (A) OI < 0.5 (n = 8), (B) OI 0.5-1.0 (n = 78), (C) OI 1.0-1.5 (n = 191), and (D) OI > 1.5 (n = 68). Intraobserver reliability was excellent for all measures: 0.89 (confidence interval: 0.87-0.91) for CW, 0.96 (0.95-0.97) for CO, and 0.97 (0.97-0.98) for OI. Interobserver reliability was good for CW 0.75 (0.70-0.79) and excellent for CO 0.90 (0.88-0.92) and OI 0.95 (0.94-0.96).

Conclusions: Variations in the morphology of the proximal femur can predispose to varus component malposition. The degree to which the GT overhangs the canal can be quantified and classified based on plain films. This can aid in preoperative planning and help guide intraoperative proximal femoral preparation.

Keywords: Anatomy; Classification; Greater trochanter; Total hip arthroplasty; Varus.

Figure 1

Technique for measuring the canal width and trochanteric overhang. The width of the diaphysis is first recorded at a point 10 cm below the lesser trochanter. Next, a line is drawn along the lateral cortex beginning at the canal width measurement and extending 4 cm proximally. A line tangential to this is drawn from the medial aspect of the greater trochanter. Finally, the distance between these 2 lines is measured.

Figure 2

When stratified by the standard deviation, the calculated OI values are normally distributed with a mean of 1.23.

Figure 4

Examples of femora with an OI of a (<0.50), b (0.50-1.0), c (1.0-1.5), and d (>1.5).

Figure 3

When preparing the proximal femur for arthroplasty, a higher overhang index predisposes to an undersized component placed in varus. Implant selection, trochanteric bed preparation, and intraoperative imaging can help mitigate this tendency.