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Figure. Side-by-side radiographs of intraoperative image and 1-month after cementless total hip arthroplasty.
A 60-year-old man with obesity presents to the office following 2 episodes of right hip dislocation. The first dislocation occurred when he was getting out of bed; while rotating his leg, he felt a “clunk” and was unable to bear weight on the leg afterword. He was taken to the emergency department where a clinician identified a hip dislocation, and he underwent a closed reduction. Two weeks after the initial dislocation, the hip dislocated again. The image shows a side-by-side comparison on an intraoperative anteroposterior radiograph during the initial hip replacement with an anteroposterior radiograph following 2 episodes of hip dislocations.
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There is increasing prevalence towards a press fit (cementless) stem for a total hip arthroplasty.1 The stem typically has a porous coating to allow bone ingrowth to the implant. The implant is wedged into the femoral canal and provides immediate stability against rotation and subsidence of the stem. The stem later becomes permanently fixed in position as the bone grows inward into the implant. A complication of cementless femoral stems is early subsidence, which can shorten the leg length and potentially cause instability of the hip.2
Subsidence generally occurs within the first 6 weeks after surgery, before biologic fixation occurs with bone ingrowth.2 Factors such as osteoporosis, body mass index, and proximal femur geometry were thought to increase the risk of subsidence; however, there is a lack of evidence to support this claim.3,4 Early weight bearing also was not shown to increase the risk of subsidence, even in patients with poor bone quality.
The risk of subsidence is correlated to the fit of the stem intraoperatively. An improper fit seen on imaging significantly increases the risk of femoral stem subsidence. Inadequate fit can be seen as a lack of implant contact on cortical bone of the proximal femur on anteroposterior radiographs. Patient weight greater than 80 kg (175 lbs) was also found to be a risk factor for early subsidence. Subsidence of greater than 10 mm or subsidence that causes instability is considered significant.1-4
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants.
References
1. Karadsheh M. THA implant fixation. OrthoBullets website. Updated May 5, 2020. Accessed October 26, 2020. https://www.orthobullets.com/recon/5003/tha-implant-fixation.
2. Ries C, Boese CK, Dietrich F, Miehlke W, Heisel C. Femoral stem subsidence in cementless total hip arthroplasty: a retrospective single-centre study. Int Orthop. 2019;43(2):307-314.
3. Rhyu KH, Lee SM, Chun YS, Kim KI, Cho YJ, Yoo MC. Does osteoporosis increase early subsidence of cementless double-tapered femoral stem in hip arthroplasty? J Arthroplasty. 2012;27(7):1305-1309.
4. Stihsen C, Radl R, Keshmiri A, Rehak P, Windhager R. Subsidence of a cementless femoral component influenced by body weight and body mass index. Int Orthop. 2012;36(5):941-947.
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