Design and Verification of a Metaphyseal Filling Stem for Total Hip Arthroplasty Based on Novel Measurements of Proximal Femoral Anatomy

Abstract

Background

Cementless metaphyseal filling stems rely on fixation in the medial-to-lateral and anterior-to-posterior (AP) planes. The purpose of this preclinical study was to develop Insignia, a new metaphyseal filling system to match the anatomy of the proximal femur, and then compare it to clinically successful stems in multiple simulations.

Methods

In this preclinical study, the geometry of the proximal femur in the AP plane among 1321 healthy subjects was evaluated using computed tomography. This data was then used to design insignia. Preclinical studies were performed to compare the broaching effort required to prepare a canal using this system, assess the reliability of seating heights for the stem, and compare in vitro micromotion testing of the stem under simulated stair climb activity.

Results

The proximal femur decreased approximately 50% in the AP plane spanning 20 mm above the lesser trochanter to 30 mm below the lesser trochanter. Additional bench top testing was performed, and the new stem system was found to demonstrate significantly reduced broaching effort (average 6 vs 29 hits, P-value = .000), reliable seating heights on stem placement, and 70% less proximal micromotion on 10,000-cyclic testing ( P < .05) compared to another clinically successful metaphyseal filling stem.

Conclusions

The AP dimension of the proximal femur decreases nearly 50% throughout its length. Metaphyseal filling stems that match the AP anatomy of the proximal femur may require fewer hits during broaching, yield reproducible seating heights, and reduce micromotion on cyclic testing.