Pure high-offset stems can cause an unexpected increase in femoral length in robotic-assisted primary total hip arthroplasty

Abstract

Background: Certain implant combinations change leg length and offset in primary total hip arthroplasty (THA). Poor restoration of leg biomechanics is a frequently cited reason for patient dissatisfaction following primary THA. A pure high-offset stem should provide direct lateralization without affecting femoral length when compared to a standard stem. However, clinical experience with preoperative planning software based on computed tomography-based three-dimensional-models reveals that using pure high-offset stems in THA may cause a difference between expected (no change in femoral length) and actual (small increase) in postoperative femoral length.

Aim: To elucidate the causes of these femoral length discrepancies using preoperative planning software.

Methods: Preoperative templating for 43 robotic-assisted THAs, optimizing acetabular size and orientation, center of rotation, stem size and offset, and prosthetic head diameter were obtained. The preoperative planning software was used to calculate differences between preoperative and postoperative femoral length for standard and pure high-offset stems, unique to each patient.

Results: Whilst the increase in femoral length between standard and high-offset stems was not significant (P = 0.93), 35 femurs (81.4%) experienced a 1-mm increase, and 3 (7.0%) experienced a 2-mm increase in femoral length while using high-offset stem compared to the standard stem. The incidence of femoral length increase was lower for patients with shorter femurs (18/22; 81.8%) compared to patients with longer femurs (20/21; 95.2%).

Conclusion: When pure high-offset stems were used in preoperative planning software, we demonstrated an unexpected increase in leg length between 1-2-mm in 88.4% of patients. This unexpected increase in femoral length is due to a function of the preoperative planning software's planned stem alignment with the anatomical axis, and not an inherent fault in the stem design. With expanding accessibility of robotic-assisted THA platforms, all potential sources of postoperative leg length discrepancy should be identified during preoperative templating and necessary alterations to the surgical plan should be made to accommodate this unexpected difference when using a pure high-offset stem.

Keywords: Anatomical mechanical femoral angle; Leg length; Robotic-assisted surgery; Three-dimensional modelling; Total hip arthroplasty.

Figure 1

Mechanical and anatomical axes of the femur. COR: Center of rotation; aMFA: Anatomical mechanical femoral axis.

Figure 2

The planned stem axis is aligned to the anatomical axis of the femur, or the expected trajectory of the femoral component of the prosthesis inside the femoral canal, and not directly aligned with the mechanical axis along which the length of the femur is measured relative to the ground. A: When using a “pure” high-offset stem, most surgeons expect the direction of change to be purely lateral. However, based on preoperative three-dimensional simulations, the actual offset direction also involves a vertical component, which has not been previously well described; B: A “pure” high-offset stem is assumed to only have a lateral offset component. However, the preoperative planning software aligns the planned stem axis with the anatomical axis, causing the centers of rotation of the standard and high-offset femoral stems to move out of the same axial plane. A component of the offset is projected along the anatomical axis, leading to a femoral length difference. Thus, using a high-offset stem will cause an increase in femoral length.

Figure 3

Effects of a “pure” high-offset stem and a plus head on the change in femoral position and leg length in a two-dimensional plane.