The Length of Diaphyseal Contact of Tapered Fluted Stems is Highly Dependent on Canal Morphology.

INTRODUCTION: Diaphyseal fixation remains the mainstay of revision THA. The stability of diaphyseal fixation can be quantified by the extent of contact between the stem and the endosteal cortex. This is highly affected by the morphology of the proximal femur. The purpose of this study was to examine factors affecting diaphyseal contact in the revision THA and to identify preoperative predictors of adequate fixation.

METHODS: Three-dimensional femur models were created from CT scans of 33 Dorr B and C femora. The proximal 120mm of the femur was omitted to mimic proximal bone deficiency. A tapered fluted stem (3 degrees, 150 mm) model was virtually implanted after reaming of the medullary canal. The contact length between stem and endosteal cortex was measured, in addition to other variables. The relationship between variables was evaluated using Spearman's correlation and logistic regression analysis was used to identify predictors of the contact length (p<0.05).

RESULTS: The contact length varied widely between specimens (66.5±16.6mm, range: 21-98mm). Contact increased with the depth of the isthmus below the lesser trochanter (range: 55-155 mm; r2 =0.473, p=0.005) and the distance between the isthmus and the distal edge of the damage zone (range: -9 - 96mm; r2 =0.508, p=0.002). Stepwise regression identified the reaming length, distance between fracture and the isthmus, and isthmus diameter as independent predictors of contact length (r= 0.643).

CONCLUSIONS: Contact is limited in specimens where the isthmus is more proximally located. In these cases, supplementary fixation using plating and/or longer, curved prosthesis may be considered.