Upon approval by the institutional review board of our hospital, we performed a retrospective analysis of relevant data from our electronic medical record system. Included in the analysis were 289 patients (322 hips) who had undergone primary THA from October 2015 to August 2018. Of them, 230 patients (involving 252 hips) had complete follow-up data. The stem used in this study was a titanium, circumferentially and proximally-coated mediolateral (ML) taper short femoral stem (Tri-Lock BPS, DePuy Synthes, Johnson and Johnson, Warsaw, IN), and was implanted with a 32- or 36-mm modular ceramic femoral head (BIOLOX Delta. The stem length (95–119 mm) increased with ML size. The acetabular component was implanted with the Pinnacle acetabular component (DePuy Synthes, Johnson and Johnson, Warsaw, IN)) in all hips. Ceramic liners (BIOLOX Delta) were used in all hips.
The inclusion criteria for this case series study were patients who had undergone THA due to osteoarthritis, acute fracture (displaced femoral neck fractures), developmental dysplasia (Crowe I or II), aseptic necrosis, avascular necrosis, drug-induced necrosis and posttraumatic arthritis, among others. The exclusion criteria were patients who were diagnosed as having hemophilic arthritis and had undergone intra-medullary nailing or total knee arthroplasty. Patients with any comorbidities causing thigh pain before the THA were not included.
All procedures were performed by three surgeons via a direct anterior (32 hips) or anterior-lateral (142 hips) or minimal invasive posterolateral approach (78 hips). The stem was inserted with a broach-only technique, and the similar broaching technique was used across the 3 surgeons. For all cases, the acetabulum was reamed to 1 mm less than the diameter of the component used. Dome screws were used to augment fixation at the surgeon’s discretion. Patients were allowed to progress to full weight bearing as tolerated, typically transitioning from a walker or crutches to a cane to no support over a period of 4 weeks.
Among the selected cases, the mean age of the patients at the time of the primary THA was 61 years (range 23–84 years) and there were 72 men and 158 women. Clinical follow-up lasted a mean time of 2.7 years (range, 1.5–4.6 years). Follow-up evaluation covered the Harris hip score (HHS) , history and examination, and determination of whether future revision surgery was planned. The HHS was obtained before operation (acute fractures not included) and at every follow-up visit. Postoperative complaints, such as thigh pain, were recorded at each visit. Whether the pain occurred at rest or during activity was not specified. If a patient demanded an explanation of “thigh pain,” she or he was told that it was pain below the hip but above the knee. If a patient reported such pain, she or he was asked whether pain was intermittent or persistent, and when it had commenced [23, 26]. The hips were divided into two groups (thigh pain group and no thigh pain group; patients who had undergone bilateral THAs could have pain in one or both hips).
Radiographs were taken within 3 days before surgery and 1 day, 6 weeks, 3, 6 months, 1 year after surgery and then on annual basis. Patients returned to the clinic for follow-up. If they were unable to return, radiographs were taken elsewhere and were sent to us for evaluation. The radiographs included anteroposterior (AP) views of the pelvis that involved the tip of the femoral prosthesis and AP and lateral views of the femur that included the hip.
All preoperative and postoperative radiographs were retrospectively analyzed, and radiological parameters were measured and checked by two authors. The following parameters were collected (Figs. 1 and 2):
(1) Pre-operative AP radiographs of the hip (Fig. 1A): (d) metaphyseal diameter 2 cm above the level of the lesser trochanter midpoint; (e) isthmus diameter which represents the width of the narrowest part of the proximal femoral canal; (f) diameter of the femoral shaft which was measured 10 cm distal to the center of the small trochanter; (g) internal width of medullar canal which was measured 10 cm distal to the center of the small trochanter. Femoral flare index (FFI) was obtained through the ratio between the metaphyseal diameter 2 cm above the level of the lesser trochanter midpoint (d) and isthmus diameter (e). Femoral cortical index (FCI) was obtained through the ratio between thickness of cortical bone (f–g) and diameter of femoral shaft (f) measured 10 cm distal to the center of the small trochanter.
(2) Post-operative AP radiographs of the hip (Figs. 1B and 2A、C): (h) the width of the stem which was measured at the proximal end of stem tip arc; (i) the internal width of medullar canal, which was measured at the proximal end of tip arc; (j) the width of the stem, which was measured at the distal end of porous coating; (k) the internal width of medullar canal, which was measured at the distal end of porous coating; (α1) coronal stem angulation (CSA), which represents the angle between the stem axis and femur axis at the first follow-up; (α2) CSA, which is representative of the angle between the stem axis and femur axis measured at the last follow-up; (s1) distance between the major trochanter apex and the stem shoulder perpendicular to the femoral stem axis measured at the first follow-up; (s2) distance between the major trochanter apex and the stem shoulder perpendicular to the femoral stem axis at the last follow-up. Stem- intramedullary canal diameter ratio (S-ICDR) at the proximal end of stem tip arc is the ratio between diameter of stem (h) and diameter of intramedullary canal (i) at the proximal end of stem tip arc. S-ICDR at the distal end of porous coating is the ratio between diameter of stem (j) and diameter of intramedullary canal (k) at the distal end of porous coating. Femoral stem subsidence (FSS) from the first to last follow-up visits is the difference between the distance at the first follow-up (s1) and the distance at the last follow-up (s2). Variation in coronal stem angulation (VCSA) from the first to last follow-up visits was obtained by subtracting the stem angulation at the first follow-up (α1) from the stem angulation at the last follow-up (α2).
(3) Post-operative lateral radiographs of the hip (Figs. 1C and 2B): (β) sagittal stem angulation (SSA), which represents the angle between the stem axis and femur axis at the first follow-up.
Stem subsidence was diagnosed when a stem subsided more than 4 mm, as measured on a perpendicular line drawn from the greater trochanter to the lateral border of the implant. And, implant loosening was diagnosed when a stem sunk more than 4 mm and/or varus/valgus migration range was greater than 5° . Stem alignment is usually defined as neutral, valgus (lateral deviation > 5°), or varus (medial deviation > 5°) . However, we did not use these values in favor of a more precise definition of stem alignment, which, we believe, is more helpful in clinical practice. The sagittal angle (on lateral radiographs) was defined as positive if the stem alignment was retroverted, and negative if it was anteverted. Similarly, the coronal angle (in AP radiographs) was deemed positive if the stem alignment was valgus, and negative if it was varus. Thus, “varus/valgus” merely reflects the extent of alignment deviation or the magnitude of stem angulation.
In previous studies, the implant fit was evaluated on the basis of the amount of implant/bone engagement as described by Wuestemann et al . In this study, we made some modifications to re-define the implant fit: the Type I fit indicates a contact between the stem tip and the adjacent cortical bone, while there has no distal contact with Type II fit. Logistic regression analysis was performed on baseline characteristics, such as implant fit, to further identify the risk factors of thigh pain.
Pearson chi-square was used for categorical variables, Student t-tests for continuous variables and logistic regression for risk factor analysis. All statistical analyses were conducted using SPSS version 20.0. A p-value < 0.05 was considered statistically significant.