Restoring global offset and lower limb length with a 3 offset option double-tapered stem

A proper restoration of joint biomechanical parameters is fundamental to achieve satisfactory outcomes in terms of hip functionality and patient’s quality of life after total hip arthroplasty (THA).

The hip abductor muscles lever arm, which is mechanically the distance between the center of rotation of the hip and the insertion point of the muscles, is in fact strictly dependent by implant positioning which determines the reconstruction of the center of rotation, as well as the global hip offset or global femoral offset (GO) [1]. GO is the distance from the anatomical axis of the femur to the medial margin of the acetabulum and it is usually defined as the sum of the femoral offset (FO) and the acetabular offset (AO) [1]. FO is defined as the distance from the center of rotation of the femoral head to the anatomical axis of the femur [2]. AO is defined as the distance from the center of rotation of the femoral head to the perpendicular line passing through the medial edge of the ipsilateral teardrop [3].

When studying hip parameters restoration after primary THA, GO is more reliable as joint parameter than considering only FO because it takes into account also the acetabular cup placement.

It has been showed how a GO reduction of more than 5 mm is associated with less abductor muscle strength and decreased functional results after THA [4]. Femoral stems are usually available with different offset versions but not always this can be a suitable solution as these different options often change, not only the stem offset but also, at the same time, the stem length and, thus, limb length (LL). A traditional cementless double-tapered straight femoral stem available with three different offset options has been shown to be able to restore hip offset even if LL discrepancy is not always minimized postoperatively [5, 6].

This study aimed to verify the restoration of the ipsilateral GO and LL radiographic parameter after primary THA by the use of a cementless femoral stem available with three different offset options which do not change the stem length. Thus, the study hypothesis, that we want to reject, is that postoperative GO decreased by more than 5 mm in comparison to preoperative GO.

Mean (SD) age at surgery of the assessed patients was 72 (8) years. Mean (SD) BMI was 26.9 (3.9), 26 patients (32%) were male and 54 patients (68%) were female. Mean (SD) follow-up was 4.4 (0.6) years, ranging between 3.2 and 5.7 years. Diagnosis was primary osteoarthritis in 63 hips (82%), avascular osteonecrosis in 8 hips (10%), femoral neck fracture sequelae in 4 hips (5%), mild hip dysplasia in 2 hips (1%), others in 2 hips (1%). Femur morphology was classified as type A in 26 hips (32%), type B in 51 hips (64%), type C in 3 hips (4%), according to Dorr classification. Mean (SD) Flare index was 3.66 (0.70), ranging from 1.92 to 5.78.

The type of Synthesis femoral stem used was A type in 1 hip (1%), B type in 59 hips (74%), C type in 20 hips (25%).

Mean GO distance significantly increased by + 3 mm after surgery (P < 0.05). ΔGO was not < − 5 mm, but within ±5 mm range, then the alternative hypothesis was accepted and the null hypothesis was rejected (Table 1).

Mean LL distance significantly decreased by − 5 mm after surgery (p < 0.05), meaning that postoperatively the limb length of the operated side increased by + 5 mm (Table 1).

GO measured preoperatively in hips treated with type B stem was significantly lower (p < 0.043) in comparison with GO measured preoperatively in hips treated with type C stem (Table 2). It means that the choice of the stem type used (type B or type C) is actually dependent on preoperative hip offset. GO measured postoperatively did not differ (p > 0.05) between hips with type B and type C stem (Table 2). Thus, the femur was lateralized after surgery at least as, or more than, the native anatomy, whatever type of stem was used.

LL measured postoperatively did not differ (p > 0.05) between hips with type B stem and those with type C (Table 2). LL was not affected by the type of stem (which actually does not modify LL) but rather depends on the size of the stem used and the positioning and size of the cup.

There were no significant differences for GO and LL between the operated side and the contralateral side after surgery, with a GO and a LL discrepancies of + 1 mm and − 0.1 mm, respectively (Table 1).

No stem subsidence was found at last follow-up. Presence of slight radiolucencies (< 2 mm) due to proximal bone remodeling was observed mostly in zone 1 in 50 hips (62%) and zone 7 in 11 hips (14%). No femoral cortical hypertrophy was seen in any radiograph. Pedestal formations were present in 2 hips. Spotwelds were noted in 19 (24%) hips predominantly in zone 3 and 5. No periprosthetic osteolysis related to polyethylene debris was seen in any radiographs.

Two dislocations occurred postoperatively in 2 (0.9%) patients. Both hips underwent revision surgery to replace the acetabular shell in one case and to replace the acetabular liner in the second case. In both revisions the femoral stem was left in situ. Other complications occurred were 1 (0.5%) intertrochanteric femoral fracture occurred intraoperatively during stem placement and treated successfully with cerclage, 1 groin pain and 1 thigh pain. The number of complications did not differ between stem offset options (Table 3).

Survival of the whole implant was 98.9% at 6-year follow-up with revision of any component for any reason as the endpoint. Survival of the femoral stem was 100% at 6-year follow-up with revision of the stem for any reason as the endpoint.

The most important finding in this paper is that a double-tapered straight stem with three different offset options can restore the native anatomy and consequently the native biomechanical hip parameters, whatever type of stem is used. Moreover, at a short- to mid-term follow-up the survivorship of the stem is excellent.

Proper restoration of GO and hip center of rotation is crucial to achieve good and long-term results. The restoration of these parameters depends on a correct surgical technique. The hip abductor muscles lever arm is in fact strictly dependent by implant positioning which determines the reconstruction of the center of rotation, as well as GO. Al-Amiry et al. [1] showed that common errors in LL are mainly caused by improper femoral stem positioning, while global FO reduction results from improper positioning of both the femoral stem and the acetabular cup.

According to Bonnin et al. [20] GO should be restored to the native anatomy. Conventional acetabular preparation consists in medialization of the cup on the acetabular floor, leading to medialization of the hip center of rotation (HCR) and decreasing AO. If surgeon compensates the lack of AO by increasing FO, this restores the GO and the abductor lever arm. Conversely, if surgeon only restores FO, without regards to the AO, GO and the abductor lever arm are reduced. Thus, the FO should be increased in order to compensate the lack of AO.

Some authors advocate the potential advantages in restoring the native HCR and therefore preserving AO [21‐23]. Potential advantages include the improvement of acetabular bone stock, lower risk of bone or soft tissue impingement and dislocation, and improved hip kinematics. Similarly, disadvantages of preserving the acetabular offset include a potential decrease of the bone coverage of the acetabular component, a higher risk of anterior overhang (a source of iliopsoas tendon impingement), and an increase of the stress applied on the cup and on the femoral head [22, 23]. Therefore, medialization of the cup gives biomechanical advantages according to Pawel’s law and gives lower resulting forces at the head-cup interface [20].

The restoration of GO should be performed in a range within ≤ ± 5 to 10 mm of native values, to gain better clinical scores, an adequate hip range of motion (ROM), gluteus strength and gait kinetics, and lower incidence of LL discrepancy [2, 4]. We agree with the authors, in fact before any surgery preoperative templating is always performed at our department, in order to choose the proper stem for each morphology of femur, limiting a LL discrepancy.

In our series GO and LL were restored within ±5 mm of the native values, leading consequently to good clinical scores. No statistically significant differences in postoperative GO and LL were found in using type B or C stems, therefore the choice of different offset options seems to be related to hip morphology.

Innmann et al. [5] in their comparative study with three stem designs stated that the postoperative LLD was surgeon but not implant specific. Junior surgeons had slightly larger LL discrepancy, compared to senior surgeons (2.4 vs 1.2 mm; P = 0.043). However for both senior and junior surgeons, leg length reconstruction was most difficult with the CLS stem with a postoperative LL discrepancy of 2.6 mm (respectively, 1.6 vs 3.6 mm) than with other types of stems, as a short curved stem (Fitmore, Zimmer-Biomet) with a postoperative LLD of 0.8 mm (P = 0.002).

Also von Roth et al. [6] in their randomized controlled trial found at 2 weeks a significant difference in postoperative LLD between patients treated with a short curved stem (Fitmore, Zimmer-Biomet) and patients with the CLS stem (respectively, − 0.7 mm and − 2.9 mm, P = .01).

The above-mentioned parameters are also related both with hip biomechanics and lower limb gait kinematics. Renkawitz et al. [24] confirmed, with their gait analysis of 60 patients with unilateral THA, that restoration of LL and GO within ±5 mm increase ROM and gait kinematics. Conversely, the authors didn’t found statistically significant distribution with the clinical scores and the restoration of LL and GO. Another gait analysis conducted by Stief et al. [25] showed that restoring GO correlate positively with the Knee Adduction Moment (KAM), so increasing offset, the knee has a greater varus moment during walking. This means that exceeding offset increase can both increase gluteus tension and hip stability, but, on the other hand, could worsen knee function. It is believed that lateral trochanteric pain may be the consequence of increased offset, but Abdulkarim et al. [26] found in their retrospective controlled case series that lateral trochanteric pain is not associated with implant positioning or increased offset.

Intraoperative femoral fractures with the Synthesis stem remain a risk during component implantation. We found 1 intraoperative intertrochanteric fracturing during stem placement. This specific complication related to this type of straight, double tapered, rectangular cross-sectional shaped design of femoral stem which appears to be reduced in our series in comparison with the classic CLS Spotorno design of which literature reported an incidence up to 4% [27, 28] not depending by learning curve, stem offset or stem positioning [29].

This study, although confirming good clinical and radiographic results with GO restoring within 5 mm range, has some limits. First, it’s retrospective with no control group. In some cases the affected hip in the preoperative radiographs were more externally rotated, due to the arthritic condition, thus the femoral neck seems more valgus and consequently the FO is decreased on the AP radiograph. In this study we compared preoperative parameters with postoperative reconstructed parameters. So, our control was the preoperative condition. It would be more interesting to have available a control group with a different design of stem, in order to directly compare the effectiveness in the restoration of GO and LL of different stem design. Finally, only two offset types of the same stem were used. Type A stem was used in only one case of hip dysplasia sequelae, with a short global offset, a mild deformity in neck anteversion and an increased CCD angle. In our series there were no valgus necks that could be matched with stem’s conformation. Probably this offset option doesn’t represent the morphology of some valgus necks, but our series is too restricted to assert it. Further prospective controlled studies with greater series are necessaries to achieve more reliable and reproducible results, but this paper shows encouraging outcomes.

The restoration of the biomechanical hip parameters after THA leads to satisfactory results. Surgical technique and preoperative planning are the first fundamental steps together with a stem with different offset options. The use of a traditional cementless double-tapered femoral stem, available with 3 offset options with no stem length changes, is effective in restoring the global femoral offset without any clinically significant change in lower limb length. Clinical results and implant survival are encouraging, despite the short follow-up.