Femoroacetabular offset restoration in total hip arthroplasty; Digital templating a short stem vs a conventional stem

Abstract

Background: Failure in restoring individual anatomy could be a reason for persistent functional limitations post total hip arthroplasty. Femoroacetabular offset (FAO) plays an important role in anatomic restoration, as loss of offset ≥ 5 mm is associated with altered gait and decreased functional outcome. Preoperative assessment by use of digital templating has shown to be a reliable method for sizing the components in total hip arthroplasty, and can show if anatomic restoration is achieved. In recent years, short stems are growing in popularity as it could allow better restoration due to more variety in placement.

Aim: To assess whether restoration of the FAO differs between a short or a conventional stem by use of digital templating. Additionally, association of the preoperative offset and caput-colllum-diaphyseal angle (CCD-angle) within restoration of both stems was investigated, and the reliability of measurements was assessed.

Methods: A total of 100 standardized hip radiographs were used for digital templating. Restoration of FAO was classified into "restored" or "not restored", when a < 5 mm or ≥ 5 mm difference from baseline value presented, respectively. Differences between the two stems concerning proportions of correct restoration of the FAO were analyzed by use of McNemar tests. To assess association between CCD-angle and preoperative FAO with absolute FAO restoration, multi-level analysis was performed by use of a linear mixed model to account for paired measurements. Through determination of the optimal point under the curve in operating curve-analysis, bootstrapping of thousand sets was performed to determine the optimal cutoff point of the preoperative FAO for restoration within the limits of 5 mm. Three observers participated for inter-observer reliability, with two observers measuring the radiographs twice for intra-observer reliability.

Results: The mean preoperative FAO was 79.7 mm (range 62.5-113 mm), with a mean CCD-angle of 128.6° (range 114.5°-145°). The conventional stem could only restore the FAO in 72 of the cases, whereas the short stem restored the FAO in all cases. CCD-angle was not a predictor, but the preoperative FAO was. A cut-off point of 81.25 mm (95% confidence interval of 80.75-84.75 mm) in preoperative FAO was found where the conventional stem was unable to restore the FAO. Reliability of measurements was excellent, with an intra-observer reliability of 0.99 and inter-observer reliability in baseline measurements higher than 0.9 between the three observers.

Conclusion: In preoperative planning of FAO restoration in total hip arthroplasty, digital templating shows that short stems with a curve following the medial calcar are potentially better at restoring the FAO compared to conventional stems if the preoperative offset is ≥ 80.0 mm.

Keywords: Anatomic offset restoration; Conventional hip stem; Offset; Short hip stem; Total hip arthroplasty.

Figure 1

Measurements in digital templating before and after hip stem placement. Arrow 1: Teardrop. Arrow 2: Femoral axis determination box. Line 1: acetabular offset. Line 2: femoral offset.

Figure 2

Femoroacetabular offset difference vs caput-colllum-diaphyseal angle for student and resident. Reference lines for varus (< 120°) and valgus (> 135°) angles. CCD: Caput-colllum-diaphyseal angle; FAO: Femoroacetabular offset.

Figure 3

Femoroacetabular offset difference vs femoroacetabular offset pre-templating for student and resident. Reference line at cutoff point of 80.5 mm pre-templating Femoroacetabular offset (FAO) for the student, reference line at cutoff point of 81.25 mm pre-templating FAO for the resident. FAO: Femoroacetabular offset.