Impact of trochanteric heterotopic ossification on measurement of femoral bone density following cemented total hip replacement.

During a study of bone mineral density changes around cemented femoral implants, we recognized heterotopic ossification occurring regularly in a position anterior to the greater trochanter and proximal femur. The aim of this study was to describe the incidence, distribution, and effect of this ossification on periprosthetic DXA scans following primary cemented total hip replacement. One hundred eleven patients underwent postoperative DXA examinations measuring changes in bone mineral density with heterotopic ossification identified and localized on standard radiographs with confirmation using DXA subtraction imaging. Male gender and age within the male group were significantly associated with occurrence of heterotopic ossification (p = 0.003 and 0.046, respectively). Femoral stem type, weight, and body mass index had no significant effect (p = 0.525, 0.372, and 0.243, respectively). Examining the Gruen zones in all patients suggested a median (plus interquartile range) zone 1 density drop of 4% (-12% to +7%). When separated and analyzed for the effect of heterotopic ossification, the 45 patients with heterotopic ossification showed a 2-year density gain of +6% (-5% to +15%), whereas those without heterotopic ossification showed a loss of 8% (-14% to 0%), a significant difference (p < 0.001). Zone 2 also showed a significant difference (p = 0.048). We therefore recommend that affected zones should be identified and excluded from analysis at all time points. Without this precaution, researchers risk underestimating periprosthetic bone loss in their studies and reporting misleading conclusions.

Assessment of inducible fracture micromotion in distal radial fractures using radiostereometry.

OBJECTIVES: This study examined the potential for measuring dynamic inducible micromotion (DIMM) between fragments in healing distal radial fractures using radiostereometry (RSA). DESIGN: Prospective imaging study. SETTING: University teaching hospital. PATIENTS: Nine patients with low-impact distal radial fractures. INTERVENTION: Volar locked plating of the fracture with insertion of tantalum beads into bone fragments. RSA examinations at 1 day and then 2, 6, 26, and 52 weeks. Motion at the fracture site was induced by maximal voluntary hand grip using a Jamar dynamometer. Radiographs were analyzed using locally developed and UMRSA software. MAIN OUTCOME MEASUREMENTS: DIMM and migration were calculated as translations and rotations of the main distal segment. Clinical precision was assessed under repeatability conditions. RESULTS: Precision (as 95% error limit) ranged from 0.06 to 0.13 mm and 0.5 to 0.8 degrees for migration, and from 0.10 to 0.14 mm and 0.6 to 1.0 degrees for DIMM. DIMM was characterized by axial and dorsal compression with dorsiflexion. The median DIMM of patients reached a maximum at 2 weeks: mainly as 0.3 mm axial compression, 0.3 mm dorsal compression, and 2.5 degrees dorsiflexion. DIMM ceased by 26 weeks, indicating union of all fractures. Fracture collapse continued until the 26-week measurement, ranging between 0.2 and 2.8 mm axially. Instability of some intraosseous markers was observed. CONCLUSIONS: The precision of this RSA method was sufficient to observe inducible movements occurring during fracture healing. This has the potential for quantifying rates of fracture union and improving understanding of the available treatments.

A hydroxyapatite graft substitute reduces subsidence in a femoral impaction grafting model.

Synthetic bone graft substitutes have been developed for impaction grafting revision hip surgery in response to concerns about the availability and potential infection risk of allograft. We performed an in vitro experiment to determine whether a synthetic porous hydroxyapatite material might improve resistance to cumulative subsidence and cyclic (elastic) movement on loading. We impacted different ratios of hydroxyapatite and allograft (0%, 50%, 70%, and 90% hydroxyapatite) into composite femoral models (eight per group) and implanted tapered, polished stems with cement. Models were loaded for 18,000 cycles on a servohydraulic materials testing machine. Migration of the femoral prostheses and the distal cement was determined using radiostereometric analysis, and cyclic movement was measured mechanically by the testing machine. Hydroxyapatite decreased overall subsidence and subsidence at the prosthesis-cement interface, but there was no difference at the cement-femur interface. This decrease may have occurred because the hydroxyapatite-containing graft was more rigid, or because it resulted in a thicker cement mantle. Cyclic prosthesis movement also was lower with hydroxyapatite. While the biologic response to porous hydroxyapatite remains to be determined, its mechanical properties appear promising.

Insertion of tantalum beads in RSA of the hip: variations in incidence of extra-osseous beads with insertion site.

Radiostereometric analysis (RSA) is a technique that can be used to measure the in-vivo micro-motion of the components of hip arthroplasty. It requires the insertion of tantalum beads into the bone permitting analysis of the radiographs. Extra-osseous beads reduce the usable bead pattern and previous studies have reported their incidence in the proximal femur as between 11% and 13% of all inserted beads. We reviewed the relative incidence of extra-osseous beads by examining 1038 radiographs of 97 patients who were part of an ongoing RSA hip study. Extra-osseous beads were seen in 44% of our patients with most having a single extra-osseous bead. The relative incidence of extra-osseous beads was 2% and 6% in the femur and pelvis, respectively. We observed a reduction in the incidence of the femoral beads with time, but not of the pelvis, which suggests that experience in the insertion technique improves the insertion rates at least of the femur. Our findings indicate that the incidence of extra-osseous beads is lower than previously reported. However, care should be taken in the design of studies to ensure that an adequate number of beads are placed in locations which are both surgically achievable and technically useful.