HIP ARTHROPLASTY

Introduction Short femoral hip stems with a metaphyseal anchoring concept have been designed to treat younger patients with good bone quality. The aim of this study was to reconstruct the centre of rotation and soft tissue balancing and preserve bone in the long-term perspective. Materials and methods Eighteen human femurs were randomised into three groups: (1) metaphyseal anchoring short stem, (2) shortened straight stem, (3) straight stem). Prior to the implantation of the hip stems, a computed tomography (CT) of the bones was performed and the femoral ante-torsion and ante-tilt was measured and compared to the results of the postimplantation CT. This could be calculated based on the 3D coordinates taken from the preand post-op CT scans, which were transformed into the same coordinate systems. Results The mean preoperative caput–collum–diaphyseal (CCD) angle for the three groups was 126.87° ± 3.50° (Group 3: 129.64° ± 3.53°, Group 1: 123.76° ± 5.56°, Group 2: 127.53° ± 1.42°) and was consistent with published reports. The postoperative CCD angles with 126.85° ± 3.43° were within a very good reconstruction range for all three groups. The anterior offset comparison among these three groups showed significant difference in reconstruction. The smallest difference between the anatomical (preoperative) and postoperative condition was seen in Group 1 (1.47° ± 0.60°), followed by Group 2 (3.60° ± 0.23°) and Group 3 (8.00° ± 0.70°) groups. The horizontal offset showed no significant difference among the groups and was within the window of ± 5 mm. Conclusion In this cadaver study, we found that the metaphyseal anchoring, partially neck-preserving short hip stem best reconstructs the ante-torsion and the ante-tilt of the femoral neck. Therefore, it can be a useful stem in younger or active middle-aged patients.


Introduction
Based on data from different national registers (e.g. Australian Register, Swedish Register), it is well known that the 10-year survival rate in patients aged < 50 years at the time of implantation in total hip arthroplasty (THA) is 88%, compared to the 98% survival rate in patients aged ≥ 75 years. Therefore, younger patients are more likely to require a revision of the hip arthroplasty [1]. Over the past two decades, short stems have emerged with the main idea of bone preservation, and numerous companies have developed different types of short stems [2,3]. These cementless femoral stems have been mainly introduced for the treatment of younger or active middle-aged patients [5,6].
However, it is important to note that only few models preserve bone in the femoral neck and are designed to have a metaphyseal anchoring concept, like the Metha® (Aesculap, BBraun Melsungen, Tuttlingen, Germany) short stem presented in this study, as compared to other shorter standard stems, for example, the Fitmore® stem (Zimmer Biomet Holdings, Warsaw, USA) that has a primary diaphyseal anchoring concept and is not meant to preserve bone in the metaphysis. In general, these short stems have still been described as bone preserving, as they are aimed at preserving the proximal bone stock and the elasticity of the femoral canal [4].
Previous studies have shown the potential advantages of metaphyseal anchored short-stem hip implants, given the reduction of stress-shielding owing to monocortical lateral contact in the diaphysis [7,8] and lesser blood loss [9] and soft tissue damage due to the smaller and curved implant during implantation [7,10]. Clinical mid-term results show comparable patient related outcome scores to standard stems [11]. Nevertheless, care should be taken when indicating the use of calcar-guided short hip stems in regards of bone quality, initial failure load and resulting migration pattern. A study of Klasan et al. showed a lower load to failure in a partially neck-preserving short stem compared to a standard stem in a biomechanical cadaver study [12].
A recent study of Jahnke et al. though showed no marked migration pattern of the same stem type after 48 months using EBRA-FCA analysis [13].
Additionally, a study by Windhagen et al. showed a more physiological soft tissue balance using Metha short stem than a standard stem because of proper reconstruction of the centre of rotation [10].
A further advantage is the preservation of bone stock, which is expected to facilitate revision procedures and enables the use of a conventional THA stem should a revision become necessary [14].
The aim of this study was to highlight that the centre of rotation, ante-torsion, and the ante-tilt were better reconstructed with a neck-preserving, metaphyseal anchored stem (Metha) compared to a shorter standard stem (CoreHip) and a standard stem (Bicontact) with a more diaphyseal fixation.

Materials and methods
18 cadaveric human fresh frozen, stripped from soft tissue femur (University Medical Center of the Johannes Gutenberg University Mainz, Germany) were used for testing. Prior to testing, a CT scan with a spiral technique CT (Somatom 4Plus, Fa. Siemens, Erlangen, Germany) was performed to analyse the ante-torsion of the femoral neck before osteotomy and after implantation of the stems. The scan protocol for each femur defined 2 volumes. One was starting from most proximal point 200 mm distal in 1 mm slices and the second volume was defined with 100 mm ending at the most distal point of the femur with slice thickness of 2 mm. Additionally, standardised radiographic images in two planes with a C-Arm (Ziehm Imaging GmbH, Nürnberg, Germany) were obtained for additional control. In consideration of this numbering, the first 3 femoral pairs (left & right) were used for the first group, the second 3 femoral pairs for the second group and the last 3 femoral pairs for the third group. Each group was assigned to one of the following hip stem type: The Metha short hip stem system used in this study is an uncemented neck-preserving monoblock (available from 2008) hip short stem. It is made of a titanium alloy with Archives of Orthopaedic and Trauma Surgery a proximal rough titanium, plasma-sprayed microporous surface coated with 20-μm dicalcium phosphate dehydrate (CaHPO4·2H2O). The four proximal trapezoidal sections allow a cortical multipoint contact in the femoral neck, while it provides a three-point fixation through the medial calcar region, the proximal lateral cortex, and the proximal posterior cortex of the proximal femur [15]. The press fit of the prosthesis types used were identical for each side 350 µm. The fixation philosophies are different for each group. While the design for Group 3 is metaphyseal fixed, Groups 1 and 2 are oriented diaphyseal. The implantation of the implants for all the three groups were performed by one experienced surgeon in all the stem designs. The femurs were fixed to the table with a vice and neck osteotomy was performed according to the surgical technique of each implant design using an electric saw (Aesculap Acculan 3Ti, BBraun Melsungen, Tuttlingen, Germany). For Groups 1 and 2, femoral neck osteotomy was performed from the trochanteric fossa at an approximate angle of 45° to the femoral axis; for Group 3, the osteotomy was performed by preserving approximately 1-1.5 cm of the lateral femoral neck with an angle of 50° to the femoral axis ( Fig. 1). After that, all stems were implanted according to the company guidelines, preoperative templating was not performed. After implantation, both CT and standard radiographic images in two planes were performed. Figure 2 provides an overview of the implantation steps.
The anterior offset of the three implanted stems was compared to the physiological situation prior to the osteotomy. The measurements were performed with the MediCad® 3D Hip Templating software program (Company Hectec, Altdorf, Germany).
The horizontal offset and caput-collum-diaphyseal (CCD) angle were calculated in the software. Additionally, the 3D coordinates of predefined landmarks (femoral head centre (1), femoral axis (2) and (6), distal femoral dorsal condyle points (8), and (9) and distal knee centre (12) were taken directly from the MediCad® software. The anterior and horizontal offset were calculated using the Microsoft Excel. Analysis of variance was used for parametric data and Kruskal-Wallis test for nonparametric data. For all analyses, p < 0.05 was considered to indicate statistical significance. Figures 3 and 4 show the images of the preoperative and postoperative CT scans with 3D reconstruction, which were then used for the above-described measurements. Figure 5 shows the anatomical landmarks used for the measurements.

Results
Tables 1 and 2 show details of the preoperative and postoperative measurements. The mean preoperative CCD angle for the three groups was 126.87° ± 3.50° and was consistent with published reports. The mean postoperative CCD angle was 126.85° ± 3.43°, within a very good reconstruction range. All groups could mostly maintain the preoperative CCD angle within a range of ± 2°. The Bicontact and Metha groups showed slightly decreased CCD angles  Archives of Orthopaedic and Trauma Surgery of 127.31° ± 2.29° and 124.5° ± 5.53°, respectively, while the CoreHip group showed an increase in the CCD angle to 128.74° ± 2.47°. None of these associations were significant ( Table 3).
The anterior offset comparison among these three groups showed a clear difference in reconstruction. The smallest difference between the anatomical (preoperative) and postoperative condition was shown by the Metha group, followed by the CoreHip and Bicontact group.
The comparison of the horizontal offset of these three groups showed a better reconstruction than the anterior offset. The average difference of anatomical and reconstructed horizontal offset was very small (0.4 mm). None of these results were significant.
The average horizontal offset of the groups was 33.76 ± 5.63 mm (25. 5-58.20). All the groups could reconstruct the anatomical horizontal offset within an average range of 5.63 mm (Table 4).

Discussion
The most important finding of this study is that a neckpreserving short stem better reconstructs the anterior offset compared to conventional straight stems.
We were able to show that the Metha short stem had the smallest change of the anterior offset compared to the preop condition of the three stems. The guidance of the femoral canal in longer stems results in a loss of anterior offset. As a neck-preserving short stem follows the canal of the femoral neck, these kinds of stems show a better reconstruction of the anterior offset.
One of the main causes of hip instability after THA is impingement that can either be bony (greater trochanter-pelvis) or implant-related (cup-neck). Dislocation of the hip joint can occur as a consequence of these impingements. To reduce the incidence of impingement, larger femoral heads or dual-mobility cup systems are widely accepted and wellinvestigated concepts [16,17]. The crucial factor to avoid impingement and optimise range of motion (ROM) in primary THA is correct implant positioning, which means an optimal combination of cup inclination, cup anteversion, and stem ante-torsion [18]. In a 3D-computed model, a theoretical formula for favourable implant positioning was postulated by Widmer et al. [19]. This study describes that for a standard total hip replacement in a vertically oriented anterior pelvic plane, the recommended cup inclination is between 40° and 42° in combination with a cup anteversion of 23° and 28°. According to the formula, a combined anteversion of cup and stem of 37.5° should be aimed for, but adjusted accordingly with respect to patient specifications.
This approach was a development based on the Ranawat's rule, proposed in the early 90s, in which a constant value of combined anteversion of the cup and stem ante-torsion of 45° in female and 20-30° in male patients was postulated [20].
Shoij et al. [16] demonstrated in a 3D CT data collection of 71 patients after THA, that a reduction of the femoral

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Bicontact Femur 1a    Table 3 The CCD angle of the anatomical, templated and reconstructed situation is shown in this table. The anterior offset is also shown in the difference of the anatomical vs.  Table 4 The horizontal offset of the anatomical, templated and reconstructed situation is shown in this Archives of Orthopaedic and Trauma Surgery ante-torsion results in a significant decrease of femoral flexion and internal rotation. The reason of this decreased ROM in their 3D model was due to bony impingement between the anterior part of the greater trochanter on the antero-inferior edge of the anterior inferior iliac spine (AIIS). In addition to the above-mentioned femoral ante-torsion measured in a transverse plane, the anterior tilt is important when speaking about restoring the centre of rotation of the femoral head.
Several studies have shown that a neck-preserving short stem like Metha properly reconstructs the centre of rotation of the femoral head and restores soft tissue balancing as a consequence [21]. An appropriate anatomical reconstruction of the hip joint in THA, besides the cup positioning, is dependent on the stem design.
In our study, we defined the centre of rotation of the femoral head preoperatively with a CT scan and evaluated the CCD angle, anterior and horizontal offset, and compared the values after implantation of the three types of stems. The Metha neck-preserving short stem showed a significantly better restoration of the native centre of rotation than the straight stems-Bicontact and CoreHip. The CoreHip stem, being a short-end straight stem, showed better values than the Bicontact stem without showing significant differences. Specifically, the values for the anterior tilt were very promising for the Metha stem, as they are close to the native values of the femur.
Our study has some limitations. First, with only six specimens per group, the sample size of this study was relatively small. Thus, this study might be underpowered. Second, this was a controlled laboratory study, and the biomechanical properties of cadaveric bone are different from an in vivo setting with respect to the soft tissue and does not reflect the conditions during surgery, postoperative ingrowth phase, and mobilisation.
Nevertheless, our results clearly underline that the centre of rotation is best restored with the Metha short hip stem. The reconstruction of centre of rotation of the femoral neck as well as the anterior offset were significantly closer to the initial parameters prior to testing. The femoral neck guidance of a short stem seems to work well to support an anatomical reconstruction, especially in the sagittal plane, and can be a suitable option especially in younger adults requiring THA.
Care should be taken though in the indication using these kind of short stems. A lower load to failure was shown in an cadaver study compared to standard stems [20]. A proper bone stock and sufficient bone quality is needed to achieve favourable results, especially in calcar-guided and monocortical diaphyseal fixation short hip stems, as presented investigated in this study. Age, BMI or prior osteonecrosis of the head does not seem to have an influence, heavy male patients nevertheless showed a higher axial migration pattern using EBRA-FCA of a calcar-guided stem (similar to Metha short stem) in a recently published clinical study in total hip arthroplasty in the treatment of osteonecrosis of the femoral head [22].

Conclusion
In this study, the reconstruction of centre of rotation of the femoral neck as well as the anterior offset were significantly closer to the initial parameters prior to testing. The femoral neck guidance of a short stem seems to work well to support an anatomical reconstruction, especially in the sagittal plane, and can be a suitable option especially in younger and active middle-aged adults requiring THA.