Abstract
Introduction: In total hip arthroplasty (THA) an optimal fit and fill of the stem is essential for stable fixation. Thus femur morphology must be studied during pre-op planning (implant choice, sizing, positioning) or when a new stem is to be designed. Using plain AP x-ray analysis and the definition of a simple two-level parameter (canal flare index, CFI), Noble et al. identified an age related transition of the endosteal canal in AP view from a ‘champagne flute’ to a ‘stove pipe’. This reference data is 2D only, limited to the endosteal geometry and the elderly age range was defined as 60–90yrs so that the number of octogenerians > 80yrs was too low to analyze morphological features of this rapidly growing and critical THA patient population.
In this study the endosteal and periosteal femur morphology of subjects > 80yrs was studied using 3D CT analysis. It was the goal to
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describe age related changes of the femur morphology in 3D,
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to study the influence of gender
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to investigate if the results may affect fit & fill of current stem designs.
Methods: High-resolution CT-scans (slice thickness 1mm) were made of 170 consenting volunteers (m/ f=101/69). The old group consisted of 119 subjects ≥80yrs (m/f=65/54, mean age: 84.1yrs [80–105]) and the young group of 51 subjects < 80yrs (m/f=36/15, mean age 67.8yrs [39–79]). After thresholding the bone boundaries in Mimics V12 (Materialise, B), the endos-teal and periosteal coordinates were analyzed for width, wall thickness, surface areas and various CFI’s relating dimensions at 20mm above LT and at a distal level (e.g. 60mm below LT, isthmus): Surface CFI (3D-CFI), frontal and lateral CFI based on the AP and ML projections (2D-CFI) and flaring in each of the four directions (1DCFI).
Results: The surface CFI was sign. lower in subjects ≥80yrs (5.08 ±1.23) than in subjects < 80yrs (6.61 ±1.72, p< 0.0001). This difference was sign. larger in females than in males (−32% vs. −17%), an observation valid with reference to any distal level. Equivalent age differences were found in both the frontal and lateral 2D-CFI as well as the medial, lateral and anterior 1D-CFI with changes in the anterior direction (−26.3%) being most dominant. In addition wall thickness was sign. reduced in the very elderly. E.g. at 20mm above LT, the medial wall measured 10.40mm at < 80yrs and 7.61 at ≥80yrs, a reduction of −27% (p< 0.001). In females (−35%) this difference was sign. larger (males: −23%, p< 0.001) even when corrected for height.
Discussion: The age driven transition of proximal femur morphology continues in the octogenarian population. This transition is not limited to two discrete levels in the AP plane as previously reported but it is a continuous 3D phenomenon with high directional asymmetry. In addition, this transition also affects the wall thickness and the periosteal shape. Furthermore a strong gender effect was identified with aging females showing increasingly and asymmetrically less flaring and thinner walls. An age and gender specific THA stem design seems necessary to fit the morphed femur. The asymmetric transition prohibits the effective use of current implant systems with proportionally scaled dimensions but favors a matrix sizing scheme with frontal and lateral dimensions changing independently.
Correspondence should be addressed to Diane Przepiorski at ISTA, PO Box 6564, Auburn, CA 95604, USA. Phone: +1 916-454-9884; Fax: +1 916-454-9882; E-mail: ista@pacbell.net
