Stem size and stem alignment affects periprosthetic fracture risk and primary stability in cementless total hip arthroplasty

Abstract

The ideal stem size and stem position is important for the success of total hip arthroplasty, since it can affect early implant loosening and periprosthetic fractures (PPF). This study aimed to investigate how small deviations from the ideal stem size and position influences the PPF risk and primary stability. Six experienced surgeons performed preoperative templating based on which the benchmark size for each femur was determined. Consecutive implantations were performed in six cadaveric femur pairs—one side was implanted with an undersized stem followed by the benchmark size and the contralateral side with a benchmark size followed by an oversized stem (Corail, Depuy Synthes). Moreover, three different alignments (six varus, six neutral, six valgus‐undersized) were compared using 18 femurs. Cortical strains during broaching and implantation were measured, and laser scans were used to determine final stem position. All specimens underwent dynamic loading. Primary stability was estimated from stem subsidence and pull‐out forces. Templated stem size varied between surgeons (±1 size; p = 0.005). Undersizing increased stem subsidence by 320% (p < 0.001). Oversized stems exhibited 52% higher pull‐out forces (p = 0.001) and 240% higher cortical strains (p = 0.056). Cortex strains increased with varus alignment (R2 = 0.356, p = 0.011) while primary stability decreased with valgus stem alignment (p = 0.043). Surgeons should be aware that small deviations from the ideal stem size and malalignments of the stem can significantly alter the mechanical situation and affect the success of their surgery.

Small design modifications can improve the primary stability of a fully coated tapered wedge hip stem

“…As younger patients rarely undergo total hip arthroplasty, this was probably connected to the high bone quality of the study group. In a previous study the same surgeon applied up to 40% lower forces to femurs with 15% less BMD during preparation of the cavities [ 33 ]. However, higher BMD was not associated with higher impaction forces during preparation or implantation.…”

“…The distal femur was sectioned 130 mm distally from the templated stem tip position and embedded in metal adapter flanges to a height of 60 mm (Technovit 4004; Kulzer, Germany). This created a consistent distance of 70 mm between embedding and the tip of the implant [ 33 ].…”

“…The implanted stems were aligned according to ISO 7206–4 with a 10°-degree lateral and 9°-degree dorsal tilt ( Fig 3A ) [ 33 ]. Sinusoidal dynamic loading with two force levels was applied using a material testing machine under force control (1 Hz; load limits of the force cell 6.3 kN, 300 Nm; Bionix, MTS, Eden Prairie, MN, US; Fig 3B ): the lower loading condition consisted of 600 cycles between 80 N to 800 N, the higher of 600 cycles between 80 N to 1600 N. A polyethylene piston served for force application ( Fig 3B ).…”

The force at the implant cannot be assessed by the mallet force–Unless supported by a model

“…To improve the force at the Points of Interest (PoI) like the implant-bone interface or the taper junction between head and stem, the force transmission from the mallet or the automated tool to the PoI must be understood. Common methods to measure the impaction force along the transmission path are instrumented mallets [4,6], instrumented broach handles [7], instrumented impactors [8], instrumented head impactors [9][10][11], load cells in the stem [5,12] or beneath the experimental setup. All of the above have in common, that the Position of Measurement (PoM) cannot be the same as the PoI.…”