Custom-Made Femoral Implants in Total HIP Arthroplasty Due to Congenital Disease of the HIP: A Review

Introduction

Congenital disease of the hip (CDH) is a common reason for the development of secondary osteoarthritis at the hip joint and the need for total hip arthroplasty (THA). Despite the fact that THA is one of the commonest operations among orthopaedic procedures, various bone and soft tissue diversities which the majority of patients with CDH present, complicate the procedure of THA. Moreover, previous operations (e.g. osteotomies) might further complicate THA. Several methods have been described till now to deal with the complex reconstruction of the acetabulum including femoral head bone grafting (1), the use of acetabular reinforcement rings (2), the proximal positioning of the acetabular component (3), the medial protrusion technique (4), the use of hemispherical cementless, small-diameter modular metal-backed acetabular components without bone grafting (5) and others. Despite the fact that all classification systems currently used in CDH classify patients according to acetabular pathology and the majority of the published literature focus on the reconstruction of the acetabulum in patients with CDH, these patients usually present with excessive anatomical diversities of the femur that require adequate preoperative planning for the optimal femoral implant selection.

Each patient with CDH represents a unique case. However, some trends of proximal femur diversities have been described including increased anteversion, shorter neck, helitorsion of the femur, decreased intramedullary canal size, thinner cortex, straight contour, and either coxavara or valga (6–7–8). The complex femoral anatomy in patients with CDH in combination with soft tissue considerations and leg length discrepancy, sometimes precludes the implantation of classical industry designed femoral stems. The use of modular (9) or cone type stems (10) has addressed the problem in several cases, where excessive rotational deformity, coxa vara and cortical thinning of the femur were not present. In cases where such deformities were seen and those where no anatomical fitting could be achieved in the preoperative planning, a customised femoral implant must be used in order to optimise the fit of the stem to the femur, to improve strain distribution and to reconstruct hip biomechanics (Fig. 1). The incidence of severely distorted proximal femoral anatomy in patients with CDH requiring a custom-made femoral implant has recently been reported to be approximately 12% (11). The present study is a review of the published literature on the use of custom-made femoral implants in patients with CDH.

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Fig. 1

Pre- and postoperative x-rays of a female patient with bilateral congenital hip disease. The patient had bilateral total hip arthroplasty with custom-made femoral stems, after extensive preoperative planning, that showed that no implant from the classical industry designs could be used in both hips in order to achieve optimal fitting and reconstruction of hip biomechanics.

Preoperative planning

Adequate preoperative planning in patients with CDH who will undergo a THA is of crucial importance for selecting the optimal implant. The published literature uses identical preoperative protocols for the assessment of patients with CDH with minor variations. These protocols are based on the principles of computer-aided design (CAD), computer-aided engineering (CAE) and computer-aided manufacturing (CAM) (12).

Initial planning involves a complete x-ray examination including an anteroposterior x-ray of the pelvis with the central beam towards the symphysis pubis, a lateral x-ray of the hip, lumbar spine x-rays and knee x-rays. A computerised tomography (CT) evaluation of the hip, femur, femoral condyle and foot is always performed with a consequent CT topogram to reveal true leg length discrepancy. A 3-dimensional (3-D) reconstruction of the femoral geometry provides data regarding both the intramedullary and the extramedullary part of the femoral anatomy, including femoral neck anteversion, centre of rotation, offset, femoral helitorsion and the transition zone between cortical and cancellous bone. The results of the CT topogram are also used to interpret the length of shortening technique (i.e. shortening osteotomy) in cases with excessive leg length discrepancies. In order to determine the optimum femoral stem, a three-dimensional reconstruction of the femoral canal using CT data and CAD is matched with a 3-D geometry of several stem designs and sizes obtained from a CAD system. The implantation of a custom-made femoral component was used in selected cases of femoral stem absence from the available designs that could perfectly fit to the femoral canal and restore the biomechanics of the hip joint.

Finally, all x-ray and CT data are processed to the manufacturer for the production of the prosthesis. The femoral stem is manufactured using a computer numerical control (CNC) machine. The implant is designed to restore the anteversion to normal of 15°. Special emphasis is given to the neck osteotomy level, the final position of the implant in relation to the lesser and greater trochanters, and the values of helitorsion and neck anteversion.

Custom-made implants

Custom implant design for the treatment of CDH is aimed at the restoration of hip biomechanics and the management of abnormal proximal femoral anatomy by the selection of the appropriate material type and geometrical form. The biocompatibility and the optimal bonding of the implant to the host bone is of crucial importance. The abnormal helitorsion (usually antetorsion) of the proximal femur is treated with the involvement of the nonsymmetrical proximal part of the stem and the neck geometry which consists of 3 plane modularity. In this manner the possibility of impingement is decreased and on the other hand, a unibody (monoblock) design of the femoral component is composed. As a result of the monoblock design, smooth transition of the loads is achieved and stress concentrations at the neck and the stem junctions, which are related to aseptic loosening, are avoided (13–14–15).

The shape of the femoral component is selected in order to cover the proximal femoral canal cavity and to provide efficient stability (Fig. 2). The main purpose is to follow canal morphology by varying the implant surface which is in contact with the bone (16, 17). This approach, known as “fill and fit”, results in the physiological distribution of stress to the femoral shaft avoiding micromotions and symptoms of thigh pain (18). Variations of the standard “fill and fit” approach have also been reported. Benum et al (19) used slightly oversized stems in initial designs and the technique involved the removal of compact bone. However, in the same centre, the diameter of later custom-made femoral stems was reduced in order to avoid distal diaphyseal fixation.

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Fig. 2

The custom-made femoral stem used by Professor Theodoros Xenakis’ team, designed by Symbios Inc.

Since the machining of the implant is mainly performed by CNC milling machines, the produced shape is a lofted circular cross section. The stem tip is designed as a hemisphere, in order to assist the implantation into the femoral canal and to avoid point loading. The material selection has to meet the requirements of biomechanical evaluation, such as strength under monotonic and cyclic loading. All reported custom-made femoral stems are made from titanium alloy. Osteointegration is achieved by surface modifications such as the augmentation of roughness and the application of coatings. The roughness of the prosthesis is provided by grid blast, an abrasive spray of particles that formulates microcavities on the surface of implant material. The coating is hydroxyapatite with a variety of covering surfaces. In the majority of custom-made femoral stems the hydroxyapatite coating is applied to the proximal part of the implant (12, 19, 20), while Flecher et al (21) used a fully hydroxyapatite coated stem. The thickness of the hydroxyapatite layer varies between 50 and 150 μm.

Surgical techniques

THA in patients with distorted anatomy is a challenging procedure, especially in cases of congenital dislocation, which is the most common reason to use a custom-made prosthesis (6). The surgeon has to address several parameters, which include the new centre of rotation, lateral off-set, femoral neck anteversion, and leg length discrepancy. It is also critical that adequate bone stock of the proximal femur is maintained, so that the stem fits well in the canal and proximal femoral stability is achieved. The use of standard femoral stems in excessively distorted anatomy precludes the achievement of these goals, leading to technical errors and suboptimal outcomes (loosening, thigh pain, abductor insufficiency). On the other hand, the use of custom-made femoral implants not only provides a perfectly fit stem to fill the distorted femoral canal, but all the above parameters can be precisely predetermined with the use of CAD-CAM technology during preoperative planning.

The same conventional surgical approaches can be used as in standard THA, depending on the surgeon's preference and the severity of the dysplasia (anterolateral, true lateral, posterior, anterior, transtrochanteric). In fact, in cases of severe dysplasia, the use of custom-made femoral stem obviates the need for trochanteric osteotomy, which might increase complication rates (22). The femoral cut height is predetermined as calculated by the CAD measurements, and precision can be maximized by special instruments provided by the manufacturer. Preparation of the femoral canal is done with one or two custom-made broaches, usually 1 mm or 2 mm smaller than the original stem. Broaches are designed to have the same shape as the stem and aim to avoid removal of but to compress cancellous bone. The respect of the cancellous bone of the proximal femur is of paramount importance. Technical errors at this step can lead to a less than optimal stem fit, or even to the inability to insert the custom-made implant.

Femoral anteversion is, as a rule, provided by the stem according to preoperative measurements, and stem insertion has to follow the native anatomy of the proximal femur. The prosthesis is designed with a neck that gives a femoral neck anteversion of either 10° (19) or 15° (12, 21) after insertion unless the surgeon decides otherwise. However, in cases of excessive femoral anteversion (>45°), it might be more biomechanically sound to perform a derotational osteotomy, either in the subtrochanteric region or distally as in cases with CDH where a standard femoral component is used (23). This is predetermined, as well as the possible simultaneous varus or valgus correction, following the guidelines from the manufacturer. The lateral offset can be incorporated into the stem, or trial modular implants can be used by the surgeon intraoperatively.

Leg length discrepancy should also be addressed, particularly in cases of high congenital dislocation, in the same manner as in cases using conventional femoral stems. Shortening osteotomies are performed in cases where leg skeletal lengthening is expected to be more than 2 cm. These osteotomies can be performed either proximally (24, 25) or distally (26, 27). Proximal osteotomies can be performed in the subtrochanteric region after canal preparation, followed by stem insertion which stabilises the osteotomy site (28, 29). However, concerns about stem stability have led some surgeons to perform a more distal osteotomy with the option of simultaneous correction of axial (varus or valgus) knee deformity (12).

Acetabular reconstruction in congenital disease of the hip is also challenging, but it is important to realize that the decision to use custom-made femoral implants does not interfere with the surgical technique of the acetabular reconstruction or the choice of acetabular implant. Various cups have been used in conjunction with custom-made stems, including press-fit hemispherical and revision cups (20, 30, 31).

Overall, the surgical technique for custom-made femoral implant is straightforward, but strict attention to detail, following the guidelines provided by preoperative planning, is critical to avoid possible technical errors.

Outcomes of custom-made prostheses

The use of cementless custom–made femoral prostheses in THA for the treatment of secondary arthritis due to CDH is especially important in young patients. The outcome of reconstruction in these patients presents new challenges, as a result of increased life expectancy and functional demand. Hence, clinical outcomes should include restoration of range of movement (ROM), quick and complete return to physical activity, long-term bone-implant fixation, and a low rate of revision.

The rarity of CDH disease along with the restricted use of custom-made components in selected cases of CDH with severe distorted anatomy of the proximal femur where no available femoral implant from the industry could be used after adequate preoperative planning, has resulted in limited studies in the published literature to date reporting on outcomes of particular implants. These outcomes should also be interpreted with caution since they refer to heterogeneous populations and implants. Each patient represents a unique case, resulting in the manufacturing of a unique implant which is difficult to compare with other implants in order to obtain reliable overall outcomes. Moreover, it should be taken into account that several studies refer to mixed populations with distorted proximal femoral anatomy that also include patients with a diagnosis other than CDH. Finally, the majority of studies do not report on long term outcomes exceeding 10 years of follow-up.

The pertinent literature was published from 2006 to 2015, with a mean follow up ranging from 6 up to 14 years and overall survival rates from 93% to 100% (11, 12, 19–20–21, 32, 33). Moreover, all studies showed statistically significant improvement in postoperative clinical scores (mainly Harris Hip Score and Merle d'Aubigne Score) compared to the preoperative period and the improvement was retained over time. Finally, no study reported any serious complications attributed to the custom-made femoral component. Intraoperative trochanteric fissure fractures that were effectively managed with wiring were reported at low rates of 1% (21), while only 1 study reported significant postoperative thigh pain in 5% of the included patients (19).

Unfortunately, in the published literature, only 3 studies reported on a homogeneous population of patients with CDH that had THA with a custom-made femoral component (11, 21, 31). In the study by Flecher et al (21) within a mean follow up of 123 months the overall survival for revision of the femoral component for any reason was 97.4% among the 97 eligible patients with CDH. Sakai et al (32) reported 4 unstable fixations among the 99 hips that were followed for more than 9 years. However, in all 4 patients, the aseptic loosening was due to intraoperative varus malpositioning of the implant. Finally, Pakos et al (11) recently published long term outcomes of custom-made femoral implants of 86 hips with CDH. Within a median follow-up of 127.5 months the 10-year survival of any of the components was 95.4% and the respective value when aseptic loosening of the stem was considered was 98.1%. Similar outcomes were reported by studies with mixed populations. Benum et al (19) in a series of 191 hips (88 with CDH) reported no case of aseptic loosening at the 7 and 10-year follow ups, while similar outcomes with no femoral stem revision at 12 years were reported by Akbar et al (20). Finally, Koulouvaris et al (12) also reported excellent outcomes of custom-made femoral stems in a mixed population of young patients, with no evidence of aseptic loosening within a mean time of 6 years.

Different standard femoral stem designs have been also used for the restoration of altered proximal femoral anatomy in patients with CDH. These stems include: (i) tapered conical or spline proximally fixated stems (34); (ii) rectangular, tapered, conical with distal fixation stems (35); (iii) cylindrical fully coated stems (36); (iv) modular designs consisting of a separate metaphyseal sleeve and a diaphyseal stem (37); (v) curved, anatomic stems that match proximal femoral endosteal geometry (38); and (vi) short-stem, bone-preserving designs (39). Both the clinical and the radiological long-term outcomes of these designs have been reported as good to excellent. Nowadays, with current extensive available designs, the use of custom-made femoral prostheses is decreasing. However, the majority of the reported problems with the use of these designs refer to excessive bone and soft tissue diversities and to patients with previous operations. Therefore, in CDH cases with severely distorted proximal femoral anatomy, these stems lack anatomical fitting, and restoration of hip biomechanics necessitating the use of custom-made implants after adequate preoperative planning.

The main disadvantages of custom-made femoral stems are prolonged manufacturing time and increased cost. For a custom-made implant, design technicians may need up to 3 months of lead time. The work flow includes interpreting CT scans, making rough prototypes of the component in clay or wax, shipping it to the surgeon, and awaiting approval or input. The whole process may be repeated until the final implant design. As far as cost is concerned, the price of a custom stem is twice or thrice as expensive as a standard stem. Finally, one should have in mind that each custom-made prosthesis is an individualised implant, designed from the beginning based on the same concept of achieving optimal proximal femoral fitting. This means that every time a new biomechanical study is performed we cannot exclude the possibility of bias.

Overall, custom-made femoral stems in CDH showed more than satisfactory outcomes in the published literature with extremely high survival and low complication rates. Unfortunately, the published literature is dominated by studies with short-term follow-up, that precludes adequate evaluation of the longevity of these implants. The rarity of the disease, combined with the rarity of patients fulfilling the criteria for a custom-made femoral stem and the extremely high cost of the implant limits the possibility of future studies to obtain more reliable outcomes.

In conclusion, optimal implant selection in patients with CDH that will undergo a THA is of paramount importance. Adequate preoperative planning is a prerequisite due to the anatomical diversities that these patients display. In cases with severely distorted anatomy of the proximal femur, where an anatomical fitting of the stem to the femur cannot be achieved via the available standard designs, a custom-made femoral stem is a valuable treatment option. The shape of the femoral component is selected in order to cover the proximal femoral canal cavity and to provide efficient stability resulting in an improvement of strain distribution and reconstruction of hip biomechanics.