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Modular Short-stem Prosthesis in Total Hip Arthroplasty: Implant Positioning and the Influence of Navigation
Abstract
Successful total hip arthroplasty with the Metha short-stem prosthesis (B. Braun Aesculap; Tuttlingen, Germany) depends on the correct indication and an accurate preoperative measurement of the femoral bone shape. Intraoperatively, bone quality, osteotomy, and implant position are of particular importance. Navigation assists with the selection of modular neck adapters for optimal free range of motion. The selection of adapters differs significantly depending on whether navigation is used during surgery. The positioning of the osteotomy depends on the surgeon’s experience and judgment as well as the local anatomical circumstances. The osteotomy can be checked intraoperatively by the positioning of the rasp. Correct positioning of the osteotomy and correct dorsolateral contact of the short stem determine the optimal implant position. Implant depth should be adjusted in relation to the lateral circumference of the femoral neck rather than in relation to the calcar osteotomy. Valgus positioning with loss of the lateral support must be avoided. Use of a double osteotomy and routine radiographic controls make it easy to implant the short-stem prosthesis in this less invasive manner. The use of navigation influences the choice of neck position toward reduced anteversion compared with non-navigated selection.
Short hip stem implants are designed for cementless fixation in the proximal femur.1 A number of these types of implants have been developed recently.2 The Metha short-stem prosthesis (B. Braun Aesculap; Tuttlingen, Germany) has a modular neck3 and is suitable for navigated implantation.4
A correct indication is a prerequisite for successful implantation of a short-stem prosthesis. In particular, the quality and shape of the bone must be considered. The use of kinematic navigation technology supports the selection of modular implant components to optimize joint reconstruction and range of motion (ROM).
This article describes the indications and contraindications for the Metha modular short-stem prosthesis (Figure 1) as well as the implantation procedure with a focus on the optimal alignment of the femoral osteotomy and the benefit of navigation for the selection of modular implant components.
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Figure 1: Postoperative radiograph shows optimal Metha stem implant placement in a 54-year-old man. Figure 2: Preoperative radiograph showing dysplasia with coxa valga and a short, wide femoral neck in a 51-year-old man. In this patient, the Metha stem could only be placed in an extreme valgus position, causing the tip of the prosthesis to move in a medial direction. Although use of the Metha prosthesis may be possible in such a case, implanting the Metha prosthesis in an extreme valgus position is not recommended. | |
Preoperative Planning
Short-stem prostheses can be implanted using a minimally invasive technique and are particularly useful in younger, active patients. Short-stem prostheses also may be useful in older patients who are interested in a less invasive procedure. However, patients must meet the necessary criteria in terms of soft tissue and body mass index (BMI), and both bone quality and bone shape must be suitable for a short-stem prosthesis. Poor bone quality or bone shapes that are not amenable to a proximal fixation of the implant are contraindications for a short-stem prosthesis in patients of all ages.
Unsuitable bone shapes include dysplastic coxarthrosis with extreme coxa valga (Figure 2) or a short, wide femoral neck and coxa vara (Figure 3). Severe antetorsion of the femoral neck also may lead to problems during implantation (Figure 4), even with a short-stem implant if the lateral radiograph projection is not considered preoperatively.
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Figure 3: Preoperative radiograph showing protrusion coxarthrosis in a 44-year-old woman with coxa vara (A). Implantation of a Metha prosthesis was not possible without lengthening the leg. Postoperative radiograph shows a straight stem was used instead with a low osteotomy and correctly adapted leg length (B). | |
A wide femoral neck (Figure 2) is a special condition to be aware of during preoperative planning, particularly when there are additional uncertainties regarding the osteotomy or the correct implant size. In such cases, an undersized implant would lead to reduced stability.
Therefore, the planning template is important in the preoperative phase. We prefer a standard prosthesis whenever a complicated reconstruction of the acetabulum is necessary because the higher osteotomy needed for the short-stem prosthesis may obstruct access to the acetabulum.
We inform our patients that our experience with the newly introduced short-stem prosthesis is still limited and that medium- and long-term results are not available. We also respect a patient’s wish to be treated with an established standard cementless implant.
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Figure 4: Preoperative radiographs showing obvious antetorsion of the femoral neck (A). Postoperative radiographs showing the tip of the Metha stem contacts the dorsolateral cortex (B). | |
Intraoperative Management
For implanting the Metha short-stem prosthesis, the anterolateral approach as described by Watson-Jones5 is particularly suitable because the muscles can be preserved. This approach is suitable with both navigated and non-navigated procedures.
As opposed to the implantation of straight stems, it is possible to drape both legs for the implantation of a short-stem prosthesis (Figure 5A), which allows the operative leg to be positioned under the contralateral leg in a “figure of four” configuration. This position aids in evaluating leg length. Because of the extreme external rotation in this position, the femoral neck will turn into the muscle compartment between the tensor fascia lata and the gluteus medius.
The navigation procedure with the OrthoPilot system (B. Braun Aesculap; Tuttlingen, Germany) supports cup and stem implantation, and allows a dedicated selection of modular neck adapters to optimize free ROM after manual implantation of the modular Metha stem. However, the femoral osteotomy and preparation with rasps is not supported by the navigation technology.
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Figure 5: Photographs showing leg position (A) and operative approach (B). Radiograph showing alignment of the two osteotomies for implanting a short-stem prosthesis (nonnavigated procedure) (C). First osteotomy: removal of femoral head (D). Second osteotomy, removal of femoral neck (E). | ||
Implantation Technique (Non-navigated)
A 7- to 9-cm incision is made in the central third between the anterosuperior iliac spine and the greater trochanter. The incision can easily be moved in a distal or proximal direction.
The incision in the fascia is basically analogous to the skin cut. If necessary, a transverse blood vessel on the anterior margin of the gluteus medius muscle can be tied (Figure 5B). After the capsule is located and the femoral head is palpated, three Hohmann retractors are placed and the capsule is almost completely removed.
For optimal positioning of the final osteotomy, it is helpful to perform two consecutive osteotomies (Figures 5C-E). The first subcapital osteotomy (Figure 5C, line 1) is performed in situ. The femoral head is removed either immediately or after the second osteotomy.
The second osteotomy (Figure 5C, line 2) is performed with the leg slightly rotated and adducted externally. The exact position of this line depends on the planned implantation depth of the stem. This second osteotomy should be slightly trapezoid in shape, with the removed bone thicker in the back than in the front. This reduces the danger of increased antetorsion of the prosthesis and makes it easier to introduce the rasps.
One important requirement for the osteotomy is that it should be closed on the lateral neck. In addition, there must be a sufficiently strong bone lamella.
After cup implantation, the leg is positioned under the contralateral leg in a “figure of four” configuration, which places the femoral neck in the optimal direction for femoral rasp preparation and stem implantation. A special Hohmann retractor supports the greater trochanter and another holds the femur to the lateral side. The femur is then opened with a small, blunt awl, which is placed in a neutral position on the osteotomy (Figure 6A). There should be no antetorsion.
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Figure 6: Photographs demonstrating implantation of the short-stem prosthesis (nonnavigated procedure). Opening of the medullary cavity (A). Femur preparation using a forming rasp (B). Preserved ring of cortex around the neck of the femur (C). Inserting the stem (D). Closure (E). | ||
The awl is pressed into the bone marrow space by hand by slightly rotating the handle. This instrument is not designed to be used in combination with a hammer; the use of the hammer in the beginning of the procedure is sometimes helpful, but not once the awl has clearly penetrated the osteotomy plane.
The awl should make contact with the lateral cortex. This is easily achieved and can be checked again when the awl is removed. One also can introduce the same awl again, thus getting a good feeling for the direction of implantation. If necessary, a second awl with twice the width can be used. This compresses the bone and offers a guide for the subsequent use of the rasps.
When using rasps of increasing size, a slight varus pressure may help to control the tendency of the instruments toward valgus. Even with the first rasp, the alignment of the osteotomy can be checked again (Figure 6B). In cases in which further resection is needed, the lateral part of the osteotomy should never be removed (Figure 6C). Proper preparation of the lateral femoral neck is necessary for assessment. The short-stem prosthesis is best implanted with a flat osteotomy (Figure 7A). A steep osteotomy can result in poor medial bone support (Figure 7B). However, if a too-low calcar osteotomy is used for orientation when implanting the stem, the stem also may placed too low, with no lateral support and a tendency to valgus (Figure 7C).
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Figure 7: Drawings showing a correct osteotomy and ideal positioning of the stem (A), a steeply angled osteotomy (B) with the stem in the ideal position, and a steeply angled osteotomy with the stem positioned too deep (C). |
During the rasping process and implantation, stem placement can easily divert from a 50° osteotomy. In addition, the shape of the bone may cause more valgus positioning of the implant. Therefore, as a rule, we exclude extreme cases of dysplastic osteoarthritis from short-stem THA.
Generally, curved short-stem rasps react differently than straight rasps. With increasing rasp size, valgus positioning may occur because the tip of the stem is guided along the dorsolateral inner side of the cortex and may shift with increasing size. Therefore, care should be taken not to lateralize the rasp during insertion.
During the rasping process, the body of the rasp should never be inserted lower than the level of the osteotomy, which would promote valgus positioning of the rasp as well. Under load, the stem could then sink below the level of the lateral resection. To avoid this problem, several extra millimeters should be left during rasping (Figure 6D). A routine check by fluoroscopy is recommended for rasp position with a trial head in situ.
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Figure 8: Photographs showing set-up for the navigated implantation of the short-stem prosthesis. OrthoPilot set up with rigid bodies (A), femoral sensor (B), c-clamp fixed to the greater trochanter (C). | ||
Navigated Implantation
For navigated implantation of the Metha short-stem prosthesis (Figure 8A), the femoral sensor (Figure 8B) can be removed (Figure 8C) and reattached on a femoral c-clamp that is firmly fixed to the greater trochanter. Trial repositioning with appropriate modular cone adapters easily allows for corrections, such as a relative antetorsion or retrotorsion of the implanted stem. This process is supported by the navigation system.
A lower centre-collum-diaphysis angle results in an effective offset increase without lengthening the leg. This is why the Metha short-stem prosthesis has some limitations in treating pronounced varus deformities.
The choice of modular trial adapters and trial head leads to an improved free ROM. The Table compares navigated and non-navigated selection of the modular neck adapter (Table and Figure 8).
The use of navigation influences the choice of neck position toward reduced anteversion. However, it should be noted the retroverted modular Metha adapters do not lead to an absolutely retroverted neck position; instead, they reduce the amount of anteversion relative to a given implant position.
Conclusion
Successful use of a short-stem hip prosthesis such as the Metha implant depends on the correct preoperative indication and bone shape. Intraoperatively, bone quality is of particular importance, and the osteotomy is dependent on the surgeon’s experience as well as the patient’s anatomy. Implant depth should never be adjusted in relation to the femoral osteotomy but rather in relation to the lateral cortex of the femoral neck. A valgus position with loss of the lateral cortical support must be avoided under all circumstances. Navigation supports the exact reconstruction of leg length, femoral offset, and ROM by improved selection of modular neck adapters.
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Figure 9: Screen display of the OrthoPilot shows navigated selection of modular Metha neck adapters. |
References
- Morrey BF, Adams RA, Kessler M. A conservative femoral replacement for total hip arthroplasty. A prospective study. J Bone Joint Surg Br. 2000;82(7):952-958.
- Gulow J, Scholz R, Freiherr von Salis-Soglio G. [Short-stemmed endoprostheses in total hip arthroplasty.] [Article in German] Orthopade. 2007;36(4):353-359.
- Buecking PK, Feldmann P, Wittenberg R. [Metha – modular short hip stem] [Article in German]. Orthop Praxis. 2006;8:474-477.
- Lazovic D, Zigan R. Navigation of short-stem implants. Orthopedics. 2006;29(10 suppl):S125-S129.
- Watson-Jones R: Fractures of the neck of the femur. Br J Surg. 1936; 23:787-808.
Authors
Dr Braun is from Vulpius Klinik GmbH, Bad Rappenau, and Drs Lazovic and Zigan are from Pius Hospital, Oldenburg, Germany.
Correspondence should be addressed to: Professor Djordje Lazovic, Klinik Für Orthopädie, Pius Hospital, Georgstraße 12, 26121 Oldenberg, Germany.
Drs Braun is a member of the B. Braun Aesculap speaker’s bureau. Dr Lazovic is a consultant for B. Braun Aesculap. Dr Zigan has not provided financial disclosure information.