Increased risk of periprosthetic femur fractures associated with a unique cementless stem design

Abstract

Background: Postoperative periprosthetic femur fractures are an increasing concern after primary total hip arthroplasty (THA). Identifying and understanding predisposing factors are important to mitigating future risk. Femoral stem design may be one such factor.

Questions/purposes: The goals of our study were to compare the (1) frequency of periprosthetic femur fracture and implant survivorship; (2) time to fracture in those patients who experienced periprosthetic femur fracture; and (3) predictive risk factors for periprosthetic femur fracture between a unique stem design with an exaggerated proximal taper angle and other contemporary cementless, proximally fixed, tapered stems.

Methods: We reviewed all hips in which a femoral hip component with a uniquely exaggerated proximal taper angle (ProxiLock) was implanted during primary THA at a single academic institution. That group of patients was compared with a cohort of patients who underwent primary THA during the same time interval (1995-2008) in which any other cementless, proximally fixed, tapered stem design was used. The two groups differed somewhat in terms of sex, age, and body mass index, although these differences were of unclear clinical significance. During the study, 3964 primary THAs were performed using six different designs of cementless, proximally fixed, tapered femoral hip prostheses. There were 736 stems in the ProxiLock (PL) patient group and 3228 stems in the non-ProxiLock (non-PL) group. In general, the stem highlighted in this study became the routine cementless stem used for primary THA for three arthroplasty surgeons without specific patient or radiographic indications. Periprosthetic fractures were identified within each group. The incidence, timing, type, and treatment required for each fracture were analyzed. The Kaplan-Meier method was used to determine study patient survival free of any postoperative fracture. Radiographs and the electronic medical record of each patient who sustained a fracture were reviewed. Followup was comparable between groups at all time points.

Results: The Kaplan-Meier estimate for fracture-free patient survival was worse in the PL group at all time points with survival of 98.4% (range, 97.4%-99.3%), 97.1% (range, 95.9%-98.3%), 95.4% (range, 93.8%-97.0%), and 92.6% (range, 89.6%-95.3%) at 30 days, 1 year, 5 years, and 10 years, respectively, for the PL patient group compared with 99.8% (range, 99.7%-99.9%), 99.6% (range, 99.3%-99.8%), 99.3% (range, 99.0%-99.6%), and 98.4% (range, 97.5%-99.1%) in the non-PL patient group (p < 0.001). Patients in the PL group had increased cumulative probability of both early and late fractures with cumulative probabilities of fracture of 2.5% (range, 1.3%-3.6%) at 90 days and 7.4% (range, 4.7%-10.4%) at 10 years compared with probabilities of 0.3% (range, 0.1%-0.5%) at 90 days and 1.6% (range, 0.8%-2.5%) at 10 years in the non-PL group (p < 0.001). Patients in the PL group had an increased risk of postoperative periprosthetic femur fracture (hazard ratio [HR], 5.6; 95% confidence interval [CI], 3.4-9.1; p < 0.001); fracture requiring reoperation (HR, 8.4; 95% CI, 4.4-15.9); p < 0.001); and fracture requiring stem revision (HR, 9.1; 95% CI, 4.5-18.5; p < 0.001). Age older than 60 years was also a risk factor for fracture (HR, 3.7; 95% CI, 2.1-6.4), but sex, body mass index, and preoperative diagnosis were not predictive.

Conclusions: Hips implanted with an uncemented femoral stem, which has a uniquely exaggerated proximal taper angle, had an increased risk of both early and late postoperative periprosthetic femur fracture. The majority of patients with a fracture underwent reoperation or stem revision. The unique proximal geometry, lack of axial support from the smooth cylindrical distal stem as well as resorption of the hydroxyapatite coating and poor ongrowth with subsequent subsidence may contribute to increased risk of fracture. Although this particular stem has recently been discontinued by the manufacturer, these findings are important in regard to followup care for patients with this stem implanted as well as for future cementless stem design in general.

Level of evidence: Level III, therapeutic study.

Fig. 1

The ProxiLock stem has a unique geometry from other proximally fixed tapered stems. The most proximal aspect of this titanium stem has a 14° circumferential flare (7° per side), which is chemically macrotextured, alumina grit-blasted, and HA-coated circumferentially. The remainder of the metaphyseal region of the stem has a 7° circumferential taper (3.5° per side), which is not chemically treated but is alumina grit-blasted and HA-coated circumferentially. The distal portion of the stem is a smooth cylinder with an alumina grit “satin” blast surface treatment and no HA coating.

Fig. 2A–B

(A) This radiograph demonstrates a late Vancouver B₂ fracture, which occurred after a ground-level fall in a patient with a ProxiLock stem. (B) When this patient underwent revision, we noted HA resorption and poor ongrowth, which were common findings in the PL group during revision for late Vancouver B₂ fractures.

Fig. 3

Cumulative probability estimates demonstrate that there was an increased probability of postoperative fracture at all time points in the PL group (solid line) when compared with the non-PL group (dashed line).