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Pre-Operative Planning
Pre-operative planning is an essential part of performing total hip replacement through a mini incision, even more so than when performed through standard approaches. Particular attention should be paid to the following areas: Patient selection. We would advise that surgeons should gain initial experience with patients whose build, diagnosis and bony anatomy are conducive to total hip replacement through a less extensive approach. Patient examination. The patient should be carefully assessed for leg length discrepancy, pelvic obliquity and fixed deformities to guide patient positioning on the operating table and the subsequent surgery. Templating of radiographs. Preoperative radiographs should be analysed using the manufacturers templates to determine the correct leg length, centre of rotation of the hip joint, femoral offset and the size of prostheses.
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Place the patient on the operating table in the lateral decubitus position. It is helpful to position the patient towards the far side of the operating table (whilst ensuring the pelvis remains vertical). This reduces the chance of impingement of the operated leg on the table, facilitating maximal adduction for subsequent femoral exposure. Ensure also that the opposite leg can be extended so that it does not oppose later adduction of the operated leg. It is advised to use ASIS supports and a sacral prop for secure and accurate stabilisation of the patient and their pelvis. This helps to ensure correct acetabular orientation. This step should be performed by the operating surgeon so that he/she is aware of the position of the pelvis in space.
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Skin Incision
The skin incision for a mini posterolateral approach is placed slightly more posteriorly and obliquely than a standard incision. The incision extends 5 cm proximally and distally to the tip of the greater trochanter. It runs obliquely over the trochanter, starting distally over the femur, passes 2 cm posterior to the tip of the greater trochanter, and continues proximally for a further 5cm.
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Deep Disection
Divide the fascia lata in line with the skin incision, extending the fascial incision 1 - 2 cm proximally and distally to the skin incision, and split the gluteus maximus muscle in the line of its fibres. This reveals the posterior aspect of the greater trochanter and the trochanteric bursa, which is reflected to expose the short external rotators. It is important to palpate the sciatic nerve, which lies posterior to these tendons, and to protect it at all times during the procedure. Internal rotation of the hip at this point will help to bring the short external rotators into view, and this is further aided by retracting the posterior border of the gluteus medius in an anterior direction. This exposes the underlying gluteus minimus tendon and the surgeon develops the plane between minimus and piriformis, which lies immediately posterior to it. Having separated these two tendons, the surgeon then detaches piriformis and obturator internus with associated gemelli from the posterior aspect of the greater trochanter.
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These may be raised with the posterior capsule as a composite flap or these two layers may be raised separately, but in either case they are repaired at the wound closure. The capsule is further split proximally in line with the original direction of the piriformis tendon and then distally as near to the trochanteric attachment as possible, and then along the posterior border of the femoral neck. A blade is passed deep to a retractor placed between gluteus minimus and the superior capsule, and used to release the superior capsule from the superior femoral neck.
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Dislocation and Neck Resection
These releases allow for easy dislocation of the femoral head, following which retractors are placed around the femoral neck to protect the soft tissues. The neck osteotomy may then be made safely.
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Acetabular Exposure
Exposure of the acetabulum may be achieved by placing the anterior retractor over the anterior column and retracting the femoral metaphysis anteriorly. This can either be stabilised by an assistant, or secured with a weight and chain.
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Release of the iliofemoral ligament and the reflected head of rectus femoris by running a knife from anterior to posterior on the ilium just above the superior acetabular rim will improve access, if required. Incising the inferior capsule down to the transverse acetabular ligament improves anterior mobilisation of the femur and assists in acetabular exposure.
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The inferior retractor is placed immediately distal to the transverse acetabular ligament, underneath the cotyloid notch. After defining the plane between the posterior capsule and acetabular labrum, one jaw of a self retaining retractor (such as a Norfolk and Norwich retractor or similar) is placed in this interval and the other jaw under the gluteus minimus muscle anteriorly. This completes the acetabular exposure. The acetabulum can now be prepared for either cemented or uncemented components in a standard fashion.
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Femoral Exposure and Preparation
To expose the femur the acetabular retractors are removed and then the non-operated leg is moved into full extension. This allows the operated leg to be placed in maximum adduction, thus delivering the proximal femur into the wound. The leg is internally rotated and a femoral elevator placed under the femoral neck. The forks of the elevator can be placed on either side of the iliopsoas tendon as it inserts into the lesser trochanter, or more proximally.
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The gluteus medius retractor is then inserted, to allow direct access to the femoral canal.
Expose the posterolateral corner of the cut femoral neck with a box chisel to allow in line access to the femoral canal. Taper pin reamers are then used to open the femoral canal. Be sure to develop exposure in the posterolateral corner to ensure central placement of the rasps and stem. The femoral canal is then prepared with the Exeter rasps.
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Ensure that there is adequate exposure to allow placement of the rasps, and subsequently the femoral stem, in the correct anteversion without soft tissue impingement on the neck of the prosthesis. This may be improved by further internal rotation of the leg, or placement of an additional retractor under the medial border of the calcar.
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After final seating of the definitive rasp a trial reduction is performed to confirm appropriate leg length, stability and choice of offset. When satisfied, mark the femur with diathermy or methylene blue dye level with the markers on the rasp, or use the leg length gauge, to guide subsequent stem insertion.
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Femoral Cementing
Measure the canal diameter and insert the appropriate cement plug. The femoral canal is thoroughly washed to remove bone fragments and fat from the endosteal surface. A suction catheter is inserted to aspirate blood from the distal canal followed by firm packing with a hydrogen peroxide soaked ribbon gauze. These are removed immediately prior to retrograde injection of cement, using a cement gun.
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The new reduced size femoral seal and extended backing plate are designed for optimum cement pressurisation through a mini incision. During stem insertion, pay particular attention to stem anteversion, as the reduced incision length can predispose to soft tissue impingement on the prosthetic neck.
Following stem insertion, cement pressurization is maintained until polymerisation, using the mini horse collar and angled stem seal pusher. It is essential that the femoral component is not allowed to move inside the cement mantle once it is fully seated.
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Reduction and Closure
Further trial reduction with the V40 trial heads can now be performed. The appropriate head is placed over the stem spigot and impacted to engage the morse taper. After final reduction the posterior capsule and short external rotators are reattached to the greater trochanter.
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