2011 Marshall Urist Young Investigator Award: when to release patients to high-impact activities after hip resurfacing

Abstract

Background: Surface replacement arthroplasties are commonly performed in young, active patients who desire return to high-impact activities including heavy manual labor and recreational sports. Femoral neck fracture is an arthroplasty-related complication unique to surface replacement arthroplasty. However, it is unclear regarding whether patients are at lower risk for fracture after a certain postoperative time.

Questions/purposes: We therefore raised the following questions: (1) does stress shielding occur after surface replacement arthroplasty, and (2) when does bone mineral density return to normal so patients can return to high-impact activities without excessive risk of fracture?

Patients and methods: We prospectively enrolled 90 patients (96 hips) with either surface replacement arthroplasty or THA, and performed dual energy x-ray absorptiometry scans at 6 weeks, 6 months, 1 year, and 2 years. We analyzed bone density by Gruen zone in both groups, and six femoral neck zones in the patients who had surface replacement arthroplasties. We calculated 6-month, 1-year, and 2-year ratios for the change in bone density compared with baseline.

Results: Bone density was greater in patients who had surface replacement arthroplasties than for patients who had THAs at 6 months and 1 year in Gruen Zones 1, 2, 6, and 7, with the largest increase in femoral neck bone density on the tension side at 6 months in Zone L1. We saw no decrease in bone density in patients who had surface replacement arthroplasties in any Gruen zone at any time, and observed no decrease in bone density in female patients.

Conclusions: Increased bone density at 6 months postoperatively in patients who had surface replacement arthroplasties provides evidence that clinically relevant stress shielding does not occur after surface replacement arthroplasty. Owing to the increased bone mineral density at 6 months, we believe patients who underwent surface replacement arthroplasties may to return to high-impact activities at that time without increased risk of fracture.

Fig. 1A–B

The radiographs show (A) the seven Gruen zones in a patient undergoing THA, and (B) the Gruen zone template that was superimposed on the image from a patient undergoing surface replacement arthroplasty to allow comparison to the seven Gruen zones in a THA.

Fig. 2

This radiograph shows the six regions of interest in the femoral neck; L1-L3 are lateral and M1-M3 are medial to the femoral neck stem.