Luxation following Press-fit Cementless Total Hip Replacement in Dogs: Novel Techniques for Predicting and Treating Post-Operative Luxation

OBJECTIVE: To report the relationship between total hip replacement (THR) luxation and soft tissue tension using acetabular (AO), femoral, (FO), global (GO) and vertical (VO) offset measurements. STUDY DESIGN: Case control study. ANIMALS: 16 dogs with THR luxation and 36 control dogs without THR luxation. METHODS: A pilot study was performed to assess the impact of pelvic rotation and varying femur position on variable measurements. Dogs with a dorsal luxation after THR were identified and controls were randomly assigned. Measurements for AO, FO, GO, subluxation index (SI) and VO were made from digital preand post-operative ventro-dorsal pelvic radiographs and the change in these variables after THR was compared between the two groups. Angle of lateral opening (ALO), femoral stem angle and subluxation index (SI) were also compared between groups. Univariate generalised linear models were used to identify variables of interest and these were assessed in multivariate logistic models. RESULTS: Relative global offset was reduced post-operatively in both groups; however, reduction was greater in the luxation group (17.7%) compared to the control group (7.4%). Decreasing GO post-operatively was associated with increased odds of luxation and GO was most affected by changes in FO. Angle of lateral opening was positively predictive of luxation. CONCLUSIONS: ALO and GO were associated with the odds of THR luxation in this cohort. CLINICAL SIGNIFICANCE: Luxation is a multifactorial and complex complication. The odds of luxation in this cohort were associated with higher ALO and reduced soft tissue tension. Isobel Monotti THR luxation in dogs 22 Introduction: Dorsal hip luxation is reported to occur in 1.1-12% of dogs after total hip replacement (THR). 1,2,3,4 Documented risk factors for luxation in veterinary patients include pre-operative coxofemoral laxity, low soft tissue tension, impingement by periarticular osteophytes and fibrous tissue and increased angle of lateral opening (ALO) of the acetabular cup. In human orthopaedics, anatomic reconstruction of the centre of rotation (COR) of the hip is an important aspect of THR planning and has been associated with patient outcome. Global offset (GO) refers to the position of the joint COR in the mediolateral plane and is closely associated with gluteal muscle tension and consequently, the stability of the hip. Global offset is the sum of an acetabular offset (AO) and a femoral offset (FO). The AO is the perpendicular distance between the centre of rotation of the femoral head and the medial acetabulum and FO is the perpendicular distance between the anatomic axis of the femur and centre of the femoral head. The bodyweight lever arm (BWLA), the distance from the COR to midline of the pelvis, together with FO can also be used to calculate GO. In humans, postoperative reduction in offset is associated with increased incidence of luxation, limping, muscle weakness, impingement, implant wear and increased joint reaction forces. Large increases in GO can result in trochanteric pain with over-tensioning of the gluteal muscles. Vertical offset (VO) refers to the proximodistal distance between the COR and the point of the lesser trochanter. In people, VO affects the resultant length of the limb and the tension in the gluteal muscles through their insertion on the greater trochanter and increases in VO after THR have been associated with reduced gluteal muscle tension. Soft tissue tension has been previously identified as an important factor for luxation in dogs, with pre-operative soft tissue laxity associated with THR luxation. A reduction in femoral displacement measurement has been associated with luxation and the protective nature of longnecked stems has been suggested. Similarly, it has been suggested that soft tissue tension could be improved with selection of implants with longer neck lengths, however, a protective relationship between increasing stem size (neck length) and luxation has not been identified. The purpose of this study was to explore the concept of offset in dogs and to determine whether these measures of soft tissue tension and COR position could be associated with THR luxation. To the authors’ knowledge, this is the first study that assesses changes in both acetabular and femoral contributions to hip COR position in relation to soft tissue tension and THR luxation in dogs. Isobel Monotti THR luxation in dogs 23 The first aim was to describe the impact of pelvic rotation and femur position on offset measurements made from extended ventro-dorsal pelvic radiographs in a normal dog. The second aim was to investigate whether changes in offset after THR were associated with the odds of luxation using a case control study. We hypothesised that a reduction in global offset after THR would be associated with luxation. Materials and Methods: Pilot Phase: Ventro-dorsal pelvic radiographs were used to assess the effect of pelvic and femoral position on offset measurements. With informed consent from the owner, a healthy, four-year-old, female spayed, 21kg Kelpie X Labrador radiographically normal hips was sedated (medetomidine 2ug/kg IV), placed in dorsal recumbency and six ventro-dorsal pelvic radiographs and one lateral projection were obtained. The pelvis was rotated to the right side by an arbitrary angle (1-5 degrees) with each subsequent view. The femurs were placed in arbitrary positions with each rotation to assess the impact of femoral rotation and abduction or adduction on measurements for femoral offset. Pelvic rotation was quantified from the radiographs using a previously described technique. Measurements of AO, BWLA, FO and VO were made from both hips in each radiograph and GO and GOBW was calculated for each position, using the methodology described below.