TOTAL HIP REPLACEMENT: TECHNIQUE
The procedure for total hip replacement begins with preoperative planning, which includes a complete medical workup of the patient to identify any existing health problems. A rheumatologist or internist often works with the orthopedic surgeon in planning the appropriate medical therapy. The rehabilitation program should also be thoroughly discussed with the patient.
The goal of preoperative planning is to create a graphic representation of a joint that will provide optimal function. The biomechanical principles that govern movement, weight bearing, and impact must be observed. The prosthetic components must be selected carefully to maximize fit and function for each individual.
|TOTAL HIP REPLACEMENT: STEPS 1 TO 3|
Lateral and anteroposterior radiographs are taken to determine the bony anatomy and the size of the femoral canal. Then clear plastic templates of each prosthesis are placed on the radiographs to choose the correct size. At this time, any existing deformities must be taken into consideration. Many patients have a limb-length discrepancy, usually a shortening of the painful limb, owing to superior displacement of the femoral head resulting from destruction of the joint space. This can be corrected with resection of the femur at the appropriate level and use of an implant with the proper neck length. If a flexion contracture of the hip exists, a more extensive dissection of the soft tissues is necessary.
The acetabulum must be evaluated for dysplasia or deficiency; in such cases, reconstruction with bone grafts, screws, or special devices is carried out (see Plate 2-49). Custom-designed implants are required if standard-sized implants are inadequate. Patients with congenital dislocation of the hip may need extra- small implants with special angles of the femoral neck.
Long-stemmed prostheses are often needed to extend past fracture sites or defect sites in the femur. If a cementless implant is used, it must fit tightly into the femoral canal and pelvis.
One of the disadvantages of the posterior approach is a historically higher rate of dislocation. However, with newer techniques—larger-diameter femoral heads and capsular repair before skin closure—this has largely been mitigated.
The direct lateral approach is another common approach to hip replacement. In this approach, the abductors are split to gain access to the hip joint. This historically allowed for a lower rate of dislocation but with a higher rate of Trendelenburg gait after the surgery because the abductors are violated.
A mini anterior approach to the hip has recently become more popular. This approach is a true muscle-sparing approach. Proponents of this approach state that because no muscles are cut, patients recover more quickly from their total hip arthroplasty. However, this has not yet been proven in clinical studies.
The patient is placed in the lateral decubitus position (supine if an anterior approach is used). A number of anatomic exposures can be used for total hip replacement, each of which has advantages and disadvantages. The posterior, or modified Moore, approach is commonly used for reconstruction of osteoarthritic hips. It allows quick and safe access to the joint without interfering with the abductor mechanism.
The limb is swabbed with iodine solution and draped to allow free movement. The bony landmarks are marked on the skin. The anterior and posterior superior iliac spine, greater trochanter, iliac crest, and shaft of the femur are all palpable.
A typical incision is centered on the greater trochanter and curves gently posteriorly, in line proximally with the fibers of the gluteus maximus muscle (see Plate 2-44). Distally, the incision overlies the femoral shaft. An incision is made in the fascia lata and the fascia over the gluteus maximus muscle. The fibers of the gluteus maximus muscle are separated proximally by blunt incision, without denervating the muscle. Care must be taken to protect the underlying sciatic nerve. The hip is internally rotated, and the piriformis tendon is identified.
A retractor is passed above the piriformis and beneath the gluteus medius and minimus muscles to delineate the superior hip capsule; another is placed deeply at the proximal border of the quadratus femoris muscle to outline the inferior joint capsule (see Plate 2-45). The piriformis and short external rotator muscles are removed from their insertions into the trochanter and stripped back to expose the posterior hip capsule, which is incised.
An anterior capsulotomy is optional. It facilitates exposure of the acetabulum and improves mobility of the femur and is desirable when the hip deformity is severe or exposure is inhibited for other reasons. In such cases, it may be helpful to expose its femoral insertion in the interval between the tensor fasciae latae and the gluteus medius muscles before making the posterior exposure.
An alternative approach is to remove the greater trochanter and reflect the attached gluteus maximus and minimus muscles superiorly for better exposure of the acetabulum. The trochanter and attached abductor muscles are turned back and held with Steinmann pins placed in the ilium, and the superolateral capsule is reflected.
After capsulotomy, the hip is ready for dislocation; it is flexed, internally rotated, and adducted to bring the femoral head and acetabulum into view. The limb is kept in internal rotation, and the insertions of the quadratus femoris muscle and the inferior capsule are incised and reflected so that the lesser trochanter can be visualized. The psoas tendon is identified but not cut. The surgeon is now ready to plan the femoral neck osteotomy.
The trial prosthesis is laid on the femur to check that the implant’s center of rotation coincides with that of the femoral head. Existing deformities of the head and neck should be corrected. Measurements upward from the lesser trochanter should be recorded for use in determining the level of the osteotomy, the desired level of the center of motion of the prosthetic femoral head, and the degree of offset. The transection line is marked on the neck of the femur, and a smooth cut is made with an oscillating saw.
The femur is retracted anteriorly to expose the acetabulum (see Plate 2-46). Unrestricted exposure of the acetabulum is the key to easy reaming and positioning of the prosthesis. Further exposure is obtained by placing a retractor posteriorly into the ischium (which also protects the sciatic nerve from injury during reaming) and another retractor inferiorly beneath the transverse acetabular ligament. The acetabulum is reamed in a medial direction to remove osteophytes and to define the true medial wall. Larger reamers are then used until the appropriate size is obtained. Trial acetabular cups are inserted, and the one with the best fit is selected. The trial cup must be positioned in the proper degrees of anteversion and of lateral inclination with the horizontal plane. Generally, the lateral inclination should be 35 to 45 degrees and anteversion should be 20 to 30 degrees. A porous ingrowth component of the appropriate size is then impacted into the acetabulum. Generally, the size of the acetabular component is 1 to 2 mm larger than the last reamer used, so that a “press fit” is obtained. One or two screws may optionally be placed to help the initial stability of the new socket.
|TOTAL HIP REPLACEMENT: STEPS 13 TO 18|
Various bearing options are available for the acetabular component. The most common option is one made of ultrahigh-molecular-weight polyethylene. Historically, total hips utilizing this material would last for 10 to 15 years. However, currently the ultrahigh-molecular-weight polyethylene is being irradiated to crosslink the polyethylene. This makes the material much more wear resistant, especially when a femoral head made of ceramic is utilized.
The final option is metal acetabular liner. This has the lowest wear rate of any available material today. However, there is a catastrophic risk of ceramic fracture. Should this occur, revision surgery is necessary.
Another disadvantage is squeaking of the hip. This is an uncommon occurrence (with a rate of 1% to 8%) but can sometimes be quite loud. Patients who develop this complication may desire revision if the squeaking is audible.
A broad retractor is placed beneath the femoral neck to bring it into clear view (see Plate 2-47). During reaming of the femoral canal, the leg is placed so that proper anteversion and valgus positioning of the stem can be achieved. The thigh is flexed, internally rotated, and adducted to achieve the best exposure of the femoral canal; a canal-finding instrument is inserted to determine the correct direction. Straight reamers and then rasps are used to shape the femoral canal. A trial prosthesis is placed in the femur, and the hip is temporarily reduced. Range of motion is tested, noting hip stability at the extremes of motion. If dislocation occurs too easily, either component may be incorrectly positioned. If soft tissue laxity contributes to instability, an implant with a longer neck may be needed.
After stable movement is demonstrated, the trial component is removed and the final implant is placed. If a cemented stem is used, a canal plug is inserted in the femur about 2 cm beyond the implant stem. This allows pressurization of the cement and prevents its flow to the distal femur. The femoral canal is again irrigated to wash out blood and debris and dried while the second batch of cement is mixed.
|TOTAL HIP REPLACEMENT: STEPS 19 AND 20|
Cement is injected into the femoral canal, then pressurized; more cement is added and then again pressurized. The implant is then inserted and correctly aligned in a neutral or slightly valgus position in the medial-lateral plane and kept in place until the cement polymerizes. Extruded cement is removed. After the wound is irrigated, the hip is reduced and once again tested for a stable full range of motion.
The posterior capsule, piriformis muscle, and conjoined tendon of the short external rotator muscles are reattached in the trochanter through drill holes (see Plate 2-48). The gluteus maximus tendon is repaired with interrupted sutures if it has been released. If a trochanteric osteotomy has been done, the trochanter fragment is replaced and secured with wires. Abductor muscle tension can be increased, if desired, by advancing the bone fragment. The deep fascia lata and gluteus maximus fascia are closed with interrupted sutures, the skin is closed with clips or a continuous nylon suture, and a sterile dressing is applied.