Femoral components used in total hip arthroplasty (THA) rely on good initial fixation determined by implant design, femoral morphology, and surgical technique. A higher rate of varus alignment may be of specific concern with short stem implants. Varus placement in uncemented femoral components has been proven not to be detrimental to clinical function; though long-term bone remodeling secondary to varus placement remains unknown. The goal of this study was to compare the clinical and radiographic outcomes in patients who underwent THA with one of two uncemented short stem metaphyseal engaging implants at minimum two-year follow-up.

A review of 105 patients (average age 65 years; BMI 29 kg/m²) who underwent a total of 109 primary THAs using the ABG II short stem femoral implant (Stryker, Mahwah, NJ), and 160 hips in 149 patients (average age 70 years; BMI 28 kg/m²) who underwent primary THA using the Citation stem (Stryker, Mahwah, NJ). The same surgeon (SDS) performed all surgeries through a less invasive posterolateral approach. Pre-operative and post-operative Harris Hips Scores (HHS) and WOMAC scores were collected. Digital radiograph analysis was performed including measuring the stem alignment relative to the femoral shaft. A stem placed with greater than 5 degrees of varus was considered to be in varus.

There was no significant difference in demographics (age, gender or BMI) or pre-operative HHS and WOMAC scores between the two groups. Follow-up HHS was 90 (range 63–100) and 94 (range 70–100) for the ABG II and Citation groups, respectively. Follow-up WOMAC scores were 10 (range 0–24) and 6 (range 0–43) for the ABG II and Citation groups, respectively. There was no statistically significant difference in any of the scores between the two groups (p>0.05).

When looking at AP radiographs for postoperative intramedullary alignment, none of the ABG II implants were placed in varus (>5°), while a small number (4.9%) of Citation implants were implanted in varus alignment. No significant difference was observed in the alignment between the two groups (p>0.05). Average post-op alignment with the ABG was 1.10° (range −4.7–4.9°) and 0.88° (range −4.5–8.9°) with the Citation.

The clinical results associated with the use of these stems in patients of all ages and bone types have been identical to those achieved by uncemented stems of standard length. Both implants in this study had excellent clinical and functional results in primary THA after a minimum 24-month follow up. In addition, postoperative radiographic analysis demonstrated that these stems can be reliably and reproducibly placed in neutral alignment despite their short length. The lateral flare on the Citation implant led to a greater number of implants in varus alignment, potentially affecting offset and leg-length, yet the relative increased incidence compared to the ABG II was not significant. Further research is needed in designing implants that optimize proximal femoral contact while maintaining alignment and overall hip kinematics.

Introduction

Many uncemented femoral implant designs have had successful outcomes in total hip arthroplasty (THA). Different uncemented stem designs achieve initial and long term stability through shape, size, coating and fit. There is increasing emphasis on bone preservation, particularly in younger and more active patients. The desire to optimize load transfer has led to the development of short stems that seek to achieve fixation in the proximal femur. Short stems designed to achieve stability by engaging the metaphysis or the proximal femoral necks are currently in clinical use. The purpose of this study was to examine the extent to which five stems designed to achieve proximal fixation contact the bone in the proximal femur. Using three-dimensional CT models of 30 femurs, we assessed the fit, fill and contact of each of the five different implants.