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Modified subvastus TKA approach offers smaller incisions, quicker recovery
Technique enhances exposure, avoids risk of denervation of inferior vastus medialis obliques.
Our experience with minimally invasive total knee arthroplasty began in 1999. We have used numerous approaches and have evolved to our current modified subvastus approach.
Minimally invasive surgery (MIS) in total knee arthroplasty (TKA) has often been defined by the quadriceps exposure. In our experience, the approaches ranked by difficulty are: 1) the mini-midvastus (most difficult), 2) mini-subvastus and 3) quadriceps-sparing (QS) approaches.
Mini-midvastus offers good exposure; however, critics argue there is risk for denervation and muscle trauma with this muscle-splitting approach. The subvastus and QS approaches claim to avoid trauma to the vastus medialis oblique (VMO) muscle, but the exposure is more challenging and may be greater risk for complications.
The goal of the modified subvastus approach is to obtain the exposure of the mini-midvastus and avoid risk of trauma and denervation of the VMO muscle while optimizing femoral exposure.
Some of the features of minimally invasive TKA surgery include a reduced incision length, a muscle-sparing approach, smaller instrumentation, non-everted patella, and in situ bone cuts.
Key steps for MIS TKA include:
- preoperative planning and templating;
- anesthesia to optimize muscle relaxation;
- an adjustable leg holder;
- reduced knee flexion;
- downsized cutting jigs;
- proximal and distal patellar releases; and
- a variable order of bone cuts to enhance exposure.
Patient selection is key, especially during the learning curve, which can be substantial. The most difficult patients are males with large muscle mass, extension loss greater than 15°, large patellar osteophytes, patella baja and Osgood-Schlatter’s lesions. The easiest patients are females with a low body mass index (BMI), supple soft tissue and good preoperative range of motion.
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Images: Bonutti PM |
Modified subvastus approach
The incision and initial exposure should be performed with the patient’s knee in 45°-60° of flexion. We recommend starting the incision 1 cm above the tibial tubercle to 2 cm above the patella, staying slightly medial to the patella and tibial tubercle. Avoid incisions over bony prominences (Figure 1). There is slightly less stretch to the skin, and patients may have an easier ability to kneel postoperatively.
Next, expose the inferior fibers of the VMO and release the fascia medially for a distance of 2 to 3 cm (Figure 2).
In the modified subvastus (Figure 3), peel the inferior portion of the muscle with the attached capsule superiorly for a distance of 1 cm. This enhances exposure to the anterior femur without risk of denervation. Next, complete the medial arthrotomy (Figure 4).
In the subvastus approach, the fascia can be released medially and superiorly to enhance quadriceps mobilization, whereas in the mini-midvastus approach, the fascia should be maintained for later closure. Next, the knee is brought into full extension to perform distal and proximal releases to mobilize the patella (Figure 5). The distal release should include a portion of the fat pad and meniscus to the anterolateral corner of the tibia (Figure 6).
The capsule underneath the VMO can either be released or resected to enhance mobilization of the patella (Figure 7).
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Fascial releases
With the knee in flexion, perform fascial releases medially and superiorly between the fat and muscle to enhance mobilization of the VMO (Figure 8).
Next, the knee is placed in approximately 70° flexion, the patella is shifted laterally, and the VMO retracted. We perform the bone cuts in this order: 1) distal femur, 2) proximal tibia, 3) 4:1 femur, and 4) patella. The order of the bone cuts can be altered to enhance exposure in the very stiff knee.
Using the downsized intramedullary instruments, we make a distal femoral cut with measured resection. Note, we do a secondary check of the Butterfly sign (Figure 9). After that, place a Homan and release the ACL, shifting the femur posteriorly (Figure 10). We use an extramedullary tibial cutting jig, cutting from an anteromedial to posterolateral.
Electrocautery can be used to deliver the tibial fragment to validate there is no retained bone, especially laterally. One can remove medial retractors and place two retractors laterally, reducing the knee flexion to resect any remaining lateral tibial bone.
Next, the knee is flexed to approximately 90°. We use the posterior referencing system, which allows several landmarks to set rotation (Figure 11). These rotational landmarks include: 1) the posterior condyles; 2) femur rotation based on the tibial cut; 3) the anterior femur/grand piano sign; and 4) the epicondyles.
We then position a downsized 4:1 cutting jig and perform the femoral bone cuts (Figure 12). We can visualize and manually palpate the edges of the bone to ensure adequacy of bone cuts and removal of residual osteophytes. These should be checked in multiple positions, both in flexion and extension.
One trick is to distract the leg in full extension. Lateral bone, osteophytes and soft tissue can be removed, which is especially useful in the valgus knee.
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Patella resection
We typically perform patella resection last to ensure appropriate osteotomy (Figure 13). Superior and inferior synovectomies from the patella can be performed to reduce the risk of postoperative crepitus. Next, we position trials, check the range of motion and track the patella.
Optimal cement technique is critical, and double-checking your tibial cut to avoid a varus position is essential for long-term survivorship. You can use an elevator to lift the femoral component to prevent scratching the femur against the tibia (Figure 14).
You should take time and check the knee both in flexion and extension, removing cement, bone and soft tissue, especially in the lateral femur and tibia.
Next, we close the knee in approximately 45° of flexion and repair the VMO muscles to medial retinaculum (Figure 15). We have used this approach on more than 1,000 consecutive primary TKAs, regardless of the patients’ age, weight or deformity.
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For more information:
- Peter M. Bonutti, MD, is an orthopedic surgeon and president of the Bonutti Clinic, 1303 West Evergreen Ave., Effingham, IL 62401; (217) 342-3400; e-mail: contactbonutti@bonutticlinic.com.