Abstract
Introduction
The Exeter cemented polished tapered stem design was introduced into clinical practice in the early 1970's. [i] Design and cement visco-elastic properties define clinical results [ii]; a recent study by Carrington et al. reported the Exeter stem has 100% survivorship at 7 years. [iii] Exeter stems with offsets 37.5–56 mm have length 150 mm (shoulder to tip). Shorter stems, lengths 95–125 mm, exist in offsets 30–35.5 mm. The Australian National Joint Replacement Registry recently published that at 7 years the shorter stems are performing as well as longer stems on the registry [iv]. Clinical observation indicates in some cases of shorter, narrower femora that fully seating a 150 mm stem's rasp in the canal can be difficult, which may affect procedural efficiency. This study investigates the comparative risk of rasp distal contact for the Exeter 150 mm stem or a 125 mm stem.
Materials and Methods
Rasps for 37.5, 44, 50 mm offset, No.1, 150 mm length stems (Exeter, Stryker Orthopaedics, Mahwah NJ) were compared with shortened length models using SOMA™ (Stryker Orthopaedics Modeling and Analytics technology). 637 patients' CT scanned femora were filtered for appropriate offset and size by measuring femoral-head to femoral-axis distance and midsection cancellous bone width (AP view). These femora were analyzed for distal contact (rasp to cortices) for 150 mm and 125 mm models (Figure 1). The widths of the rasp's distal tip and the cancellous bone boundary were compared to assess contact for each femur in the AP and ML views; the rasp was aligned along an ideal axis and flexed in order to pass through the femoral neck (ML view only).
Results
The sample size of appropriate patients totaled 238 femora. In the AP view, the rasp exhibited contact in 43 cases for a 150 mm stem but in 0 cases for a 125 mm stem; 95% of bones with contact were Champagne Fluted. In the ML view, rasp distal contact occurred in 52 femora for a 150 mm stem and in 1 femur for a 125 mm stem (Table 1). The difference was significant in both views with p < 0.001.
Discussion
This study shows that a shortened stem design's rasp avoids distal contact. Shorter stem rasps resolved all cases where there was a risk of contact with a 150 mm rasp and reduced the likelihood of contact (one case compared to 52), AP and ML views respectively. These results indicate that shorter stems may address patients with champagne-fluted and/or excessively bowed femora, commonly found in the Asian population[v]. Contact avoidance may improve rasp seating height (AP view) and alignment with the femoral axis (ML view), thereby increasing procedural efficiency and producing an optimal cement mantle distally.[vi] The data shows that a total 29% of appropriate model patients would benefit from a shorter stem. Shorter cemented stems may effectively address the global population's needs in THR.
