Abstract
Background
Short femoral stems were introduced for total hip arthroplasty (THA) to preserve bone stock, increase proximal load transfer, and enable minimal invasive approaches. However, after implanting the short curved Fitmore hip stem, an increased occurrence of cortical hypertrophy (CH), cortical thinning (CT) and radiolucency (RL) was observed. There are few long-term data on the progression of CH with the Fitmore hip stem, and no data on other radiological parameters. Furthermore, the clinical relevance of these radiological parameters (e.g. for thigh pain) has not yet been investigated.
Methods
80 Fitmore hip stems were evaluated prospectively over 10-years. The main radiographic parameters were CH, CT, RL and subsidence. The clinical parameters were thigh pain, Oxford Hip Score and Harris Hip Score.
Results
The rate of CH was 69% after 1 year, and 74% after 10 years. CH mainly appeared in Gruen Zones 2, 3, 5, and 6. CT was 65% after 1 year, and 80% after 10 years, mainly in Gruen Zones 6, 7, 8, 9, 12, and 13. RL was found in 58% of cases after 1 year and in 18% of cases after 10 years, primarily in Gruen Zones 1 and 8. Subsidence was 1.6 ± 1.6 mm after 1 year and 5.0 ± 3.1 mm after 10 years. The Oxford Hip Score increased from 22 to 43, the Harris Hip Score from 59 to 94. The prevalence of preoperative of thigh pain was 96%, dropping to 16% after 10 years. Over 10 years, the radiological parameters did not correlate with any of the clinical parameters.
Conclusions
Over 10 years, the short curved Fitmore hip stem did not prevent proximal stress shielding and exhibited CH and CT. However, this had no clinical consequences and was not correlated with symptoms. Despite its primary press-fit fixation, we observed that, unlike other stems, the Fitmore hip stem did not settle to its final position after one year, but continued to subside over 10 years by a maximum of 5 mm.
Keywords: Total hip arthroplasty, Fitmore, Clinical results, Radiographic results, Long-term, Short stem, Subsidence, Cortical hypertrophy, Thigh pain
Background
Short femoral hip stems were introduced into total hip arthroplasty (THA) to preserve bone stock by transferring load more proximally (thus avoiding proximal stress shielding), optimize future revision if needed, and enable minimal invasive approaches [1–6]. The Fitmore hip stem (Zimmer Biomet, Winterthur, Switzerland) is a short, uncemented, curved femoral stem. It achieves primary stability through a press-fit fixation and a triple-tapered design [1, 2]. This stem achieves rotational stability through contact with the calcar region and its trapezoidal cross-section. The Fitmore hip stem has three primary points of fixation: the calcar proximally Gruen Zone 7, the lateral endosteal surface Gruen Zone 2 and 3, and the medial tip Gruen Zone 5. Once settled with some subsidence, there is a 2-point contact distally with increased load transfer. Short- and mid-term clinical outcomes showed excellent results, with improvements in the Harris hip score (HHS) from 60 preoperatively to 99 after 2 years [7] and from 59 to 94 after 5 years [8]. Survival rates of 100% after 2 years [5] and 99% after 5 years [8] were reported.
The Fitmore hip stem induces remodeling of the proximal periprosthetic bone structure by acting more rigidly than long stems. This results in radiographically detectable alterations such as cortical hypertrophy (CH), cortical thinning (CT) and radiolucency (RL) [2, 8–10]. Values ranging from 29 to 71% have been reported the literature for the rate of CH after 1–5 years, primarily in Gruen Zones 3 and 5 [5, 8, 11, 12]. No other radiological parameters, nor their progressions over time, have been reported for Gruen Zones. There are also no data on how these parameters relate to stem settlement as measured by subsidence progression.
The clinical significance of the aforementioned radiological changes remains unclear. No clinical differences were found between patients with or without CH after 1–5 years [5, 8, 11]. Data on other radiological parameters, e.g. cortical thinning or radiolucencies, are lacking. Furthermore, clinical outcomes were only measured using scores (e.g. Oxford Hip Score or Harris Hip Score), and the correlation with thigh pain was never investigated.
The primary aim of this study was to evaluate the progression of radiological parameters in a cohort treated with the short curved Fitmore hip stem over a long-term period of 10 years. The secondary aim was to reveal any associations between these radiological parameters and clinical outcomes.
Materials and methods
This prospective study was approved by the local Ethics Review Board, and all patients provided written informed consent. The study was conducted in accordance with the Declaration of Helsinki and applicable legislation. Between April 2008 and April 2010, a total of 123 primary THAs were performed on 120 patients using the Fitmore hip stem (Zimmer Biomet, Winterthur, Switzerland, see Fig. 1).
Fig. 1.
Radiographs in the anteroposterior (ap) and axial planes of the Fitmore hip stem position in the femoral shaft. Load transfer contact points at the implant-bone interface are indicated via arrows at the femoral head element, calcar, and anterior/posterior subtrochanteric shafts. Gruen Zones are numbered in red. The subsidence (S) is marked in blue. These radiographs show typical radiological changes of the Fitmore hip stem after 10 years, including cortical hypertrophy in Gruen Zones 3 and 5, cortical thinning in Gruen Zones 6 and 7, and the absence of radiolucencies
The inclusion criteria were treatment with the Fitmore hip stem within the time frame of the study and the availability of complete clinical and radiographic data, including data from the preoperative and all postoperative follow-up visits (after 1, 2, 3, 5, and 10 years). Patients were excluded if they were lost to follow-up (n = 14), refused to participate (n = 12), died (n = 10 patients with 11 operated hips), required a stem-related revision (n = 1; conversion to a CLS stem due to aseptic loosening), or had data missing (n = 5). Therefore, at the final 10-year follow-up, 80 Fitmore hip stems (78 patients: 30 female, 48 male) were suitable for evaluation. The average age of the patients was 61 with a standard deviation of ± 10 years (60 ± 11 years for female and 60 ± 10 years for male). The mean BMI was 27.3 ± 4.8 kg/m2. Indications for THA were primary osteoarthritis (92%), fracture (3%), osteonecrosis (2%), post-traumatic arthritis (2%), and hip dysplasia (1%).
Surgery and postoperative protocol
The surgical interventions were carried out by a small team of orthopedic surgeons in a large general hospital. This ensured consistency in surgical techniques, management of pre- and postoperative treatment, and anaesthesiological procedures across all patients. The Fitmore hip stem was used as the primary standard implant in all cases, except where there were contraindications due to decreased bone quality (Dorr type C) on the preoperative radiographs.
Depending on the surgeon’s preference, the surgical approaches were either minimally invasive anterolateral (80% of cases) or direct lateral (Hardinge, transgluteal) (in 20% of cases). A Fitmore cup was used in 90% of cases, while a trabecular metal modular cup with a CoCr head (Zimmer Biomet, Winterthur, Switzerland) was used in the remaining 10% of cases due to low bone quality found during surgery. The following acetabular liners were used: Alpha Durasul Standard 110 (89%); Trilogy Longevity, XLPE, 3.5 mm Offset, 10 Degree Elevated Rim 10 (8%); Trilogy Longevity, XLPE, 3.5 mm Offset 2 (2%), and Alpha Sulene PE 1 (1%). All patients underwent full weight-bearing activities with a 4-point crutch gait immediately after surgery.
Prospective clinical evaluation
Patient data (age and gender, BMI) and implant data (stem size and type A, B, or C) were recorded at the start of the study. Clinical data were collected at baseline, immediately postoperatively, and during follow-up after 1, 2, 3, 5, and 10 years, and included a physical examination, incidence of thigh pain, and Harris Hip and Oxford Hip scores.
Radiographic evaluation
Antero-posterior (AP) and axial radiographs with the legs turned inwards were taken at each visit. To standardize the radiographs, a 25 mm radiopaque gage ball was positioned between the patient’s thighs (at baseline), and the diameter of the implant head was used based on the size of the type (for postoperative radiographs).
The 10-year follow-up radiographic measurements were performed using a PACS-Web-Viewer program (GE Healthcare, Solingen, Germany). All radiographs were evaluated by two different investigators, and a third independent surgeon reviewed the measurements on which they disagreed (ICC 0.97). The 10-year follow-up included the following parameters: (1) cortical hypertrophy (CH), i.e., the distance from the inner to the outer side of the cortical bone vertical to the stem axis; (2) bone condensation, which appears as a denser area on radiographs due to the trabecular bone reacting to stem implantation, typically below the tip of the stem; (3) cortical thinning (CT); (4) radiolucency (RL); (5) reactive lines; (6) calcar rounding; (7) calcar resorption; (8) subsidence, which manifests itself in AP radiographs as the difference between the implants’ shoulder level and a parallel line of the tip of the greater trochanter (immediate postoperative radiographs were used as a reference point for the follow-up radiographs) (Fig. 1); and (9) varus/valgus position, i.e., the angle formed by stem’s axis, as determined by the most distal point of the stem and the midpoint between stem shoulder outer stem neck and the femur. The definition of the neutral position was at 0° ± 5°. Higher positive values were considered as varus, and higher negative values as valgus.
Statistics
R programming (version 4.2.1) was used for the data analysis, with the mean and standard deviation are reported [13]. Patient parameters, radiographic parameters (Table 1), and clinical outcome parameters (thigh pain, Oxford Hip Score) were analyzed descriptively. The effect of radiographic parameters on clinical outcome was analyzed: Binary outcome parameters assessed in AP and axial radiographs were collapsed into a single variable. “Yes” was recorded if either AP or axial in one Gruen Zone were “Yes”; “No” was recorded if both AP and axial were “No”. Categorical variables were compared using a chi-square test and a Fisher exact test, and continuous variables were compared using either a Wilcoxon rank sum test (for two groups) or a Kruskal–Wallis test (for more than two groups). Kendall’s rank correlation was computed for two ordinal variables. No correction for multiple tests was made in this exploratory study. The level of statistical significance was set at 0.05 for all tests.
Table 1.
Radiographic results after 10 years follow-up
| Radiographical parameter | 1 year | 2 years | 3 years | 5 years | 10 years |
|---|---|---|---|---|---|
| CH | 69% | 68% | 70% | 70% | 74% |
| Bone condensation | 79% | 95% | 97% | 98% | 100% |
| Cortical thinning | 65% | 69% | 73% | 73% | 80% |
| Radiolucency | 58% | 42% | 37% | 22% | 18% |
| Reactive lines | 44% | 35% | 29% | 17% | 10% |
| Calcar rounding | 70% | 74% | 76% | 82% | 86% |
| Calcar resorption | 8% | 8% | 6% | 4% | 4% |
| Mean subsidence [mm] (± SD) | 1.6 (± 1.6) | 1.8 (± 1.7) | 1.9 (± 1.8) | 2.0 (± 1.9) | 5.0 (± 3.1) |
Of the large amount of radiological data collected during 10 years of follow-up, we only report the CH, CT, and RL. These they are the most important parameters and cortical thinning also serves as proxy for some of the other variables, most notably calcar rounding and osteolysis.
Results
10-year results are summarized in Table 1.
Cortical hypertrophy
One year after surgery, the rate of CH was at 69%, increasing to 74% after 10 years (Table 2). CH mainly occurred on the AP radiographs in Gruen Zones 2, 3, 5, and 6 (Table 2; Fig. 1). CH on axial radiographs was always less than 10% in all Gruen Zones. On AP radiographs, the maximum thickness of CH after 1 year was 10.6 ± 2 mm on the medial cortex and 9.6 ± 2 mm on the lateral cortex. This increased by 1.8 mm and 1.4 mm, respectively, over 10 years. On axial radiographs, the maximal thickness of CH after 1 year was 9.1 ± 2.3 mm on the ventral cortex and 8.9 ± 3.3 mm on the dorsal cortex. Both of these increased by 2.6 mm and 0.8 mm, respectively, over 10 years.
Table 2.
Appearance of CH in Gruen zones over 10 years. Frequencies are in relation to the total number of patients. The mean distances are calculated for the group of patients with CH
| Cortical Hypertrophy | 1 y | 2 y | 3 y | 5 y | 10y |
|---|---|---|---|---|---|
| AP: Y / N | 68.8% | 67.5% | 70% | 68.8% | 73.8% |
| AP Gruen Zone 1 | 2.5% | 2.5% | 2.5% | 1.3% | 1.2% |
| AP Gruen Zone 2 | 38.8% | 36.2% | 38.8% | 36.7% | 28.7% |
| AP Gruen Zone 3 | 58.8% | 57.5% | 57.5% | 58.2% | 60% |
| AP Gruen Zone 4 | 1.2% | none | none | none | 6.2% |
| AP Gruen Zone 5 | 43.8% | 45% | 47.5% | 46.8% | 56.2% |
| AP Gruen Zone 6 | 18.8% | 20% | 21.2% | 20.3% | 23.8% |
| AP Gruen Zone 7 | none | none | none | none | none |
| Mean medial thickness (SD) | 10.6 (± 2.2) | 10.8 (± 2.3) | 10.7 (± 2.3) | 10.6 (± 2.3) | 12.4 (± 2.7) |
| Mean lateral thickness (SD) | 9.6 (± 1.9) | 9.6 (± 2.0) | 9.5 (± 2.0) | 9.5 (± 2.0) | 11.0 (± 2.3) |
| axial: Y / N | 11.2% | 10% | 8.8% | 11.2% | 8.8% |
| axial Gruen Zone 8 | 2.5% | 2.5% | 1.3% | 2.5% | 2.5% |
| axial Gruen Zone 9 | 5% | 3.8% | 2.6% | 3.8% | 3.8% |
| axial Gruen Zone 10 | 1.2% | 1.3% | 1.3% | 1.3% | 2.5% |
| axial Gruen Zone 11 | none | none | none | none | none |
| axial Gruen Zone 12 | 6.2% | 6.3% | 3.8% | 6.3% | 5% |
| axial Gruen Zone 13 | 5% | 3.8% | 3.8% | 5.1% | 3.8% |
| axial Gruen Zone 14 | none | none | none | none | none |
| Mean ventral thickness (SD) | 9.12 (± 2.3) | 9.88 (± 2.17) | 9.14 (± 1.95) | 9.56 (± 2.19) | 11.71 (± 3.25) |
| Mean dorsal thickness (SD) | 8.88 (± 3.31) | 9.12 (± 3.87) | 8.57 (± 3.1) | 9.11 (± 3.33) | 9.71 (± 3.2) |
Cortical thinning
We found that CT was 65% after one year, increasing to 80% after 10 years. In the AP view, CT was predominantly present in Gruen Zone 2, 6 and 7, though it could also be seen in Gruen Zones 1 and 3 (Table 3). Over 10 years, the appearance of CT increased by 10–32% across all affected Gruen Zones. In the axial view, CT was primarily seen in Gruen Zone 8–9 and 12–13, and occasionally in Gruen Zones 10 and 14. CT increased by 15–25% over 10 years in all affected Gruen Zones.
Table 3.
Appearance of cortical thinning in Gruen zones over 10 years. Frequencies are in relation to the total number of patients
| Cortical Thinning | 1 y | 2 y | 3 y | 5 y | 10y |
|---|---|---|---|---|---|
| AP: Y / N | 47.5% | 55% | 58.8% | 56.2% | 71.2% |
| AP Gruen Zone 1 | 2.5% | 6.2% | 7.5% | 7.6% | 7.5% |
| AP Gruen Zone 2 | 2.5% | 5% | 6.2% | 8.9% | 35% |
| AP Gruen Zone 3 | none | none | none | none | 2.5% |
| AP Gruen Zone 4 | none | none | none | none | none |
| AP Gruen Zone 5 | none | none | none | none | none |
| AP Gruen Zone 6 | 7.5% | 13.8% | 15% | 13.9% | 28.7% |
| AP Gruen Zone 7 | 43.8% | 50% | 53.8% | 51.9% | 53.8% |
| axial: Y / N | 43.8% | 56.2% | 61.3% | 66.2% | 70% |
| axial Gruen Zone 8 | 35.4% | 40.5% | 42.3% | 49.4% | 41.8% |
| axial Gruen Zone 9 | 12.7% | 21.5% | 21.8% | 26.6% | 30.4% |
| axial Gruen Zone 10 | 1.3% | 1.3% | 5.1% | 3.8% | 6.3% |
| axial Gruen Zone 11 | none | none | none | none | none |
| axial Gruen Zone 12 | 3.8% | 7.6% | 7.7% | 12.7% | 19% |
| axial Gruen Zone 13 | 10.1% | 19% | 23.1% | 22.8% | 35.5% |
| axial Gruen Zone 14 | 2.5% | 3.8% | 5.1% | 5.1% | 3.8% |
Radiolucency
RL was 58% in the first year, decreasing to 18% after 10 years. After 1 year, RL appeared mainly in Gruen Zones 1 (39%) and 8 (23%) and then almost disappeared in all Gruen Zones over 10 years, reaching values of ≤ 10% (Table 4).
Table 4.
Appearance of radiolucency in Gruen zones over 5 years. Frequencies are in relation to the total number of patients
| Radiolucencies | 1 y | 2 y | 3 y | 5 y | 10 y |
|---|---|---|---|---|---|
| AP: Y / N | 50% | 36.2% | 28.7% | 18.8% | 17.5% |
| AP Gruen Zone 1 | 38.8% | 32.5% | 27.5% | 19% | 10% |
| AP Gruen Zone 2 | 2.5% | 3.8% | 3.8% | 2.5% | 1.2% |
| AP Gruen Zone 3 | 11.2% | 6.2% | 2.5% | 1.3% | 6.2% |
| AP Gruen Zone 4 | 13.8% | 8.8% | 5% | 1.3% | 7.5% |
| AP Gruen Zone 5 | 16.2% | 7.5% | 3.8% | 1.3% | 6.2% |
| AP Gruen Zone 6 | 11.2% | 3.8% | 3.8% | 2.5% | 3.8% |
| AP Gruen Zone 7 | 15% | 8.8% | 7.5% | 3.8% | 1.2% |
| axial: Y / N | 35% | 22.5% | 18.8% | 11.2% | 5% |
| axial Gruen Zone 8 | 22.8% | 16.5% | 15.4% | 11.4% | 3.8% |
| axial Gruen Zone 9 | 3.8% | 7.6% | 3.8% | 3.8% | 2.5% |
| axial Gruen Zone 10 | 11.4% | 10.1% | 7.7% | 1.3% | 0% |
| axial Gruen Zone 11 | 10.1% | 7.6% | 6.4% | 1.3% | 1.2% |
| axial Gruen Zone 12 | 11.4% | 10.1% | 6.4% | 1.3% | 0% |
| axial Gruen Zone 13 | 6.3% | 8.9% | 5.1% | 5.1% | 2.5% |
| axial Gruen Zone 14 | 13.9% | 10.1% | 7.7% | 8.9% | 2.5% |
Subsidence
The mean Subsidence was 1.6 ± 1.6 mm after 1 year, increasing by 0.4 mm to 2.0 mm after 5 years. After 10 years, it had increased by a further 3 mm, reaching 5.0 ± 3.1 mm.
Other radiological parameters
Bone condensation increased from 79% after one year to 98% after 5 years and 100% after 10 years. Reactive lines were observed in 44% of cases after 1 year, 17% after 5 years, and 10% after 10 years. Calcar rounding increased from 70% after 1 year to 82% after 5 years, and to 86% after 10 years. Conversely, calcar resorption decreased from 8% after 1 year to 4% after 5 years, with no further changes observed thereafter.
Clinical parameters
The 10-year survival rate was 99% (with 1 revision due to aseptic loosening). The Harris Hip Score increased from preoperative values of 59 ± 16 to 94 ± 7 after 10 years, while the Oxford Hip Score rose from preoperative values of 22 ± 9 to 43 ± 6 after 10 years. The Oxford severity of pain reported no pain in 52% of patients, while 48% experienced pain ranging from very mild to severe (very mild 21%, mild 22%, moderate 4%, severe 2%). Similarly, the majority of patients reported no or at most moderate pain on the Eq. 5D-severity scale (none 70%, moderate 27%, excessive 3%). The majority of patients (86%) experienced no thigh pain 10 years after surgery. Only 14% of the patients experienced slight or moderate thigh pain (slight 13%, moderate 1%, severe 0%).
Complications
Complications included 1 hip dislocation, which occurred on the first postoperative day after mobilization and was treated with closed reduction; 1 postoperative haematoma on the 11th day, which was resolved with conservative therapy; 1 anterior impingement, which was treated with an exchange of the acetabular cup; and 1 deep wound infection 7 days after surgery, which was successfully treated with open debridement, irrigation, and antibiotics, without the need to remove the implant.
Correlation
There was no correlation between the radiographic parameters listed in Table 1 and the clinical outcomes over 10 years, except for subsidence. The subsidence was significantly larger (by 2 mm) in patients with thigh pain after 2 and 3 years (p < 0.01 and p = 0.01, respectively), but after 5 years (p = 0.12) and 10 years (p = 0.85) this difference could no longer be detected and all patients had the same subsidence. The main parameters – cortical hypertrophy, cortical thinning, and radiolucency – had no clinical significance when accounting for the Oxford Hip Score, the Harris Hip Score, the Oxford Hip Score-pain or thigh pain. Neither patient characteristics (age, gender, or BMI) nor implant characteristics influenced either clinical or radiological outcomes.
Discussion
This long-term study prospectively documents a cohort of 80 short curved Fitmore hip stems over 10 years. It is the first study to report radiological parameters (focused on CH, CT and RL) detailed by Gruen Zones, tracking progression over 10 years. It is also the first study to investigate the clinical impact of these parameters.
The Fitmore hip stem was originally designed to have three primary points of bone contact: calcar proximally Gruen Zone 7, lateral endostal surface Gruen Zones 2 and 3, medial tip Gruen Zone 5. However, after settling down with some subsidence, it had only two points of bone contact: distally with increased load transfer resulting in distal CH and proximal CT. In this study, CH mainly appeared on antero-posterior radiographs in Gruen Zones 2, 3, 5, and 6 with CH rates of 69% after 1 year and 74% after 10 years. The literature reports CH rates of 29–63% after 1–3 years, 71% after 5 years, and 26−56% after 10 years for the Fitmore hip stem [5, 11, 14, 15]. Freitag et al. reported an occurrence rate of CH of 26% and a mean subsidence of 1.4 mm after 10 years, hypothesizing that the incidence of CH was a result of subsidence [15]. This could explanation our higher occurrence of CH [15]. Innmann et al. reported a higher rate of CH of 54% after a mean of 7.7 years, finding a correlation between the incidence and the reconstruction of hip offset [14]. In comparison, the Optimys hip stem showed only a CH rate of 4.5% after up to 7 years, with CH being localized in Gruen Zones 3 and 5 [16, 17]. This can be explained by differences in the stem design: the Optimys hip stem was developed to avoid distal fixation and to reduce the incidence of thigh pain [16, 17]. However, our findings show that CH has no impact on either the clinical outcome or thigh pain, which is consistent with other studies [14, 18–22].
This study is one of the few to report on the settling of the Fitmore hip stem over a 10 year period. Subsidence was found to be 1.6 mm after 1 year, increasing by 0.4 mm to 2.0 mm after 5 years and a further 3 mm to 5 mm after 10 years. This rate is higher than that reported by others, who only found a mean subsidence of only 1.4 mm with the Fitmore hip stem after 10 years [15]. Rilby found the subsidence of the Fitmore hip stem to be of 0.7 mm in the first 3 months, with no further migration thereafter during up to five years of follow-up [23]. However, our subsidence rate is consistent with that reported in other studies involving both short and long stems, which found subsidence rates of 1–4 mm [7, 24–27]. There are several possible explanations. First, as previously mentioned, the Fitmore hip stem was designed to have three points of bone contact. However, after settling down with subsidence of the implant, it ended up with only two points of bone contact. Other stem types differ in that an initial remodeling is primarily observed within the first 6–12 months, with little to no further change after 3–4 years [20, 28, 29]. Second, Mittelstaedt et al. reported higher subsidence in a subgroup of of younger and male patients [30]. One possible explanation for the increased subsidence observed in our study group after 5 years may be the higher proportion of young and male patients. Third, Freitag was the first to describe a correlation between CH and subsidence [15]. Our finding of relatively large subsidence of 5 mm and a relatively high percentage of CH of (74%) are consistent with Freitag’s findings [15]. However, our results contrast with those of Cho et al. [19] who reported that subsidence in hips with CH was less than in hips without CH (1.5 versus 3.4 mm). Fourth, poor bone quality could be a reason for subsidence; however, we excluded Dorr type C, and the average age of female and male participants was 60 years, which is relatively young.
Different scores measure pain in different ways. To avoid inconsistencies, we used the Oxford Hip Score to measure the pain severity throughout, and we specifically evaluated “thigh pain” with an additional question at 1, 2, 3, 5, and 10 years. The Oxford Severity of pain score was 95% preoperatively and 48% 10 years after surgery. We found that neither patient characteristics (age, gender or BMI) nor implant data or radiological data influenced the severity of pain in the Oxford Hip Score. Thigh pain has been attributed to distal femoral stem fixation [16, 17]. Despite the Fitmore hip stem having a 2-point fixation at the distal tip of the stem, resulting in a high rate of CH, no correlation between CH and thigh pain could be observed. In this study the preoperative thigh pain was 96%, dropping to 14% 10 years after surgery. This is comparable to thigh pain reported after Fitmore (24%) or CLS-stem (18%) implantation [23]. We found no influence of patient characteristics (age, gender or BMI), implant data or radiological outcomes on thigh pain, which is consistent with other findings [18–22]. It remains to be determined which patients will experience pain following the implantation of the Fitmore hip stem. Even excluding differential diagnoses such as lumbar back pain or iliosacral osteoarthritis by narrowing the question from “do you have pain?” to “do you have thigh pain” did not allow us to identify a relevant factor. Further research is necessary to clarify this point.
The study has several strengths. First, its prospective design differentiates it from the majority of other publications, which are retrospective. Second, with a follow-up period of 10 years, it is one of the longest ever reported in the literature [5, 11, 15]. Third, it is based on the most detailed radiological data divided by Gruen Zones ever reported after 10 years for the Fitmore hip stem, or indeed any other short curved femoral stem.
The study has some limitations. First, the sample size is relatively small, primarily due to patients being lost to follow-up, refusing to participate, or dying. Second, a holistic assessment of the results of total hip replacement would also include an evaluation of acetabular cup and liner wear, not just the femoral shaft [31–33]. Third, the surgical approach was not standardized, which could affect the outcome. Fourth, the method used to measure migration could be considered a limitation. We have referenced all measurements to the diameter of the implant head. However, EBRA (Single-Image-Radiograph-Analysis) or RSA (Radiostereometric analysis) could provide more precise measurements [23], with RSA being considered one of the most reliable and well-established methods for measuring migration.
In conclusion, over a period of 10 years, the short curved Fitmore hip stem did not prevent proximal stress shielding and exhibited CH and CT. However, this was without clinical consequences, and there was no correlation with symptoms. Despite its primary press-fit fixation, we observed that, unlike other stems, the Fitmore hip stem did not settle to its final position after 1 year; instead, the subsidence continued over 10 years. However, this was also without clinical consequence, and no correlation with other symptoms such as thigh pain was observed. This suggests that other contributing factors may be relevant when pain persists. Identifying these factors is a challenge for future research.
Acknowledgements
Thanks to Caroline Thalmann and Jana Schader for the help in the data collection.
Author contributions
AF contributed to the study design; KSM and TS to clinical and radiological data collection; AF to statistical analysis; KSM and AF to the writing; TS to the editing. All authors have read and approved the final manuscript.
Funding
Open access funding provided by University of Basel
Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon request.
Declarations
Ethics approval and consent to participate
This study was approved by the local Ethics Review Board (KEK-NR 132 − 13) and all patients provided written informed consent. The study was conducted in accordance with the Declaration of Helsinki and applicable legislation.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used and/or analyzed during the current study are available from the corresponding author upon request.