Minimally invasive total knee arthroplasty is a new and changing technique that has potential advantages over conventional total knee arthroplasty. This chapter will describe the advantages, disadvantages, and contraindications of this procedure, elucidate the most common approaches, illustrate the results from the past 10 years, and explore future developments.
Possible advantages include decreased pain, improved range of motion, greater quadriceps strength, quicker recovery, and greater patient satisfaction.
Possible disadvantages include increased technical difficulty, longer operative time, and higher frequency of component malposition as well as joint malalignment.
Five main approaches have been described, each with advantages and disadvantages.
Computer-assisted navigation combined with minimally invasive surgery may improve outcomes.
New instruments and technologies are being developed to increase the performance and success rates of this procedure.
Careful patient selection is important.
Familiarity and experience with the procedure may improve results.
Proper instruments designed for each approach are crucial.
The incision/exposure can be expanded as needed to improve visualization.
Vary the exposure window by moving all retractors together to view all the aspects of the joint.
Verify proper joint alignment and femoral rotation.
Avoid excessive traction to the skin, quadriceps mechanism, and patella.
Be aware of the locations of neurovascular structures relative to the approach.
Avoid eversion of the patella as this may weaken the extensor mechanism.
Avoid transcortical placement of tracking pins for computer-assisted navigation to reduce the chance of iatrogenic fracture.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
Total knee arthroplasties (TKAs) have been progressively changing since the 1950s. Surgeons continually look for ways to improve component design as well as instrumentation. This has resulted in improvements in range of motion and overall function as well as success rates ranging from 90% to 100% at up to 14 years postoperatively. However, the most common technique used to approach the knee joint and place the prosthesis has changed little since the 1940s. This technique involves larger incisions often up to 12 inches long, disruption of the patellar tendon as well as the medial retinaculum, eversion of the patella, dislocation of the tibiofemoral joint, and further injury to the surrounding soft tissue. The instruments used for the procedure, although precise, have been relatively large and thus can also contribute to extensive tissue destruction.
During the past 10 years, various changes have been made to the surgical technique in an attempt to reduce the trauma to the surrounding structures, termed minimally invasive surgery (MIS). No standardized definition of MIS has been formally established for TKAs. However, the most common aspects include smaller incisions, smaller instruments, decreased injury to the quadriceps muscle and other soft tissues, as well as avoidance of patellar eversion and tibiofemoral joint dislocation. Other factors that have been modified for MIS techniques include patient positioning, incision location, and extent as well as placement of soft tissue release. Earlier and faster rehabilitation regimens have been included with some MIS techniques.
Several possible advantages of MIS-TKA have been reported. Studies have found decreased postoperative pain, shorter hospital stays, earlier ambulation, improved range of motion in the early postoperative period, and reduced rehabilitation time relative to conventional TKA. Other studies have suggested that MIS-TKA may lead to quicker improvement of quadriceps muscle strength and function. Patient perception of better cosmetic appearance has also been reported. All of these benefits may subsequently lead to higher patient satisfaction with the outcome.
A number of disadvantages of MIS-TKA have also been described. The procedure involves a limited view and decreased access to the joint, so it is thought to be more technically difficult than conventional TKA. This has been associated with longer operative times, less optimal joint alignment, and higher frequencies of component malposition. All of these possible disadvantages may lead to decreased function and higher failure rates. However, these risks may decrease in frequency with increased experience, careful patient selection, and improved techniques. Some of these procedures have shown substantial learning curves, while others may have smaller learning curves of only 10 patients. Nevertheless, MIS procedures are relatively new, so long-term success rates have not been evaluated.
Indications for undergoing MIS TKA are similar to those for conventional TKA. Patients who have disabling pain, limited function, or deformity as a result of a knee disease such as end-stage osteoarthritis or osteonecrosis and who have not responded to more conservative treatments are candidates for this procedure.
Several contraindications to MIS TKA have been enumerated, and thus careful patient selection is important to try to ensure that each patient is getting the most beneficial treatment. Patients who have underlying cardiac, respiratory, vascular, immune, or other diseases that make them poor surgical candidates should be made aware of these risks, especially as MIS TKA may involve longer operative times than conventional procedures. Surgeons have found that an MIS approach may not be possible for patients who have poor soft tissue structure due to rheumatoid or inflammatory arthritis, diabetes, chronic corticosteroid use, or prior surgery. Patients who are overweight or who have very thick quadriceps muscles may also require more soft tissue dissection for accurate component placement. Additionally, patients who have significant joint deformity or limited knee motion may require more removal of soft tissue to restore proper alignment or movement. As a result, it is important that patients understand that MIS is not always the most appropriate procedure, and that intraoperative conversion to a conventional surgery is sometimes necessary.
Several approaches have been described for MIS TKA. The main goals of such approaches include a smaller incision, less soft tissue destruction, preservation of the extensor mechanism, as well as less discomfort and faster recovery of the patient. Three of these approaches have been used multiple times by several authors. These are the mini-medial parapatellar approach, the mini-subvastus approach, and the mini-midvastus approach. A fourth approach, the mini-lateral approach, is a newer approach that has been studied in one published report at this time. Additionally, computer-assisted navigation has been used in combination with MIS TKA in an attempt to improve success rates and functional results.
The mini-midvastus approach was one of the earliest approaches described for MIS TKA. The knee is placed in flexion and the skin incision is made from the tibial tubercle to the superior pole of the patella, passing medial to the patella, as shown in Figure 14-1 . The capsule is opened along the line of the incision, and a 2-cm cut of the vastus medialis obliquus muscle (VMO) is made proximally in line with the muscle fibers at the level of the superior pole of the patella. This cut can be extended further if necessary. Figure 14-2 shows the capsular cut for this approach, as well as for other approaches discussed in this chapter. Inferior and superior capsular releases are performed until the patella can be subluxed laterally to expose the joint space. Retractors are placed to allow maximum exposure, as shown in Figure 14-3 . A common practice during MIS TKA is to move all of the retractors in a synchronized manner to displace the exposure window medially, laterally, proximally, or distally to view all aspects of the joint. Care must be taken to avoid excessive pulling on the retractors to avoid damage to the skin and soft tissues. The tibial and femoral bone cuts can be performed with the knee in 60 to 70 degrees of flexion in a similar manner to conventional TKA, as shown in Figure 14-4 , except that the cutting guides are smaller to fit the exposure window. Flexion to greater than 90 degrees and external rotation of the joint can allow the proximal end of the tibia to protrude toward the opening for placement of the tibial component. The joint can be held in distraction for placement of the femoral component, as shown in Figure 14-5 . The knee can then be reduced and examined for range of motion and balance. If those parameters are acceptable, the final components can be cemented into place. Special care must be taken apply the cement properly and remove all excess cement, which can be difficult to visualize through the small incision. All structures, including the VMO, are then closed with the knee in flexion.
The main advantage of the mini-midvastus approach is reduced damage to the quadriceps mechanism, as a result of both avoidance of patellar eversion and cutting the VMO in line with its fibers rather than transversely. The primary disadvantages that have been cited include the difficulty of the procedure, and the hypothetical risk of denervation of the inferior body of the VMO below the incision, which has not been documented.
Mini-Medial Parapatellar Approach
The mini-medial parapatellar approach, another commonly used technique, is similar to a conventional knee arthroplasty approach but with a smaller incision. The patient is placed supine on the operating table, with the knee maintained in extension. A 10- to 14-cm skin incision is made from the superior pole of the patella to the tibial tubercle. The incision may curve medially around the patella or may pass superficially over the medial aspect of the patella. After the fascia is released, medial and lateral flaps can be constructed. The knee is then flexed to allow greater exposure. The quadriceps tendon is then cut longitudinally from the superior pole of the patella proximally, separating the medial third from the remainder of the structure. This dissection is extended until the patella can be subluxed laterally to expose the joint without eversion. Following this, the bone cuts can be made and the components can be placed as described in the previous approach.
This approach has some reported advantages and disadvantages. It is similar to the conventional approach, so it may be less difficult for surgeons to perform relative to other MIS approaches. It is purported to provide adequate exposure to all aspects of the joint, as well as avoid proximity to nerves and blood vessels. Additionally, if further exposure is needed, the incision can be lengthened and the patella everted, resembling a conventional arthrotomy. Disadvantages include disruption of the patellar tendon, which may increase the chances of scarring or later rupture.
The mini-subvastus approach was designed to avoid directly cutting through any aspect of the quadriceps muscle or patellar tendon, to preserve function of the extensor mechanism. It begins with a midline (or slightly medial) skin incision made to a flexed knee, extending 10 to 14 cm proximally from the medial aspect of the tibial tubercle. The fascia is then opened and elevated. The joint capsule is opened medial to the patellar tendon, extending from the tibial tubercle proximally to the middle of the patella. The inferior border of the vastus medialis muscle is bluntly separated from the medial retinaculum and associated connective tissue, without violating its attachment to the patella. This separation is continued until the patella can be subluxed. The bone cuts and component placements are then performed as described previously. Closure of the soft tissues is straightforward.
The mini-subvastus approach has several advantages and some potential disadvantages. It avoids disruption of the quadriceps muscle and patellar tendon, possibly decreasing the likelihood of rupture and increasing the recovery and strength of the extensor mechanism postoperatively. It also avoids eversion of the patella. Avoiding these may lead to decreased postoperative pain, greater flexion, earlier ambulation, and higher quadriceps muscle strength relative to the conventional approach. The most concerning potential disadvantage is the risk of injury to neurovascular structures in close proximity to the subvastus incision, specifically the musculoarticular branch of the femoral artery and the saphenous nerve.
Mini-Medial Quadriceps-Sparing Approach
The mini-medial quadriceps-sparing approach is a technique that was first described for unicondylar knee arthroplasty in 2004, but has recently been adapted for TKAs. With the knee in flexion, a 10- to 14-cm skin incision is made from the superior pole of the patella around the medial aspect of the patella to the tibial tubercle. A capsulotomy is made medial to the patellar ligament from the insertion of the vastus medialis muscle to the insertion of the patellar tendon on the tibia. The vastus medialis muscle is left intact if possible, but may need to be cut if it has a low insertion on the patella or if further exposure is needed. The patella can then be moved laterally to expose the joint space. Following this, the bone cuts are made, the components are placed, and the closure is straightforward.
The primary advantage of this approach is that it attempts to avoid damage to the extensor mechanism, which may result in increased quadriceps muscle strength postoperatively. However, the authors acknowledge that to obtain sufficient exposure, the vastus medialis muscle may need to be cut near its insertion, thus negating the quadriceps-sparing aspect, or a transverse incision may need to be made inferior to the vastus medialis muscle, which would convert it to a subvastus approach. Other disadvantages include the difficulty of the exposure and procedure if the vastus medialis muscle is not cut, which may result in suboptimal alignment or component placement. Additionally, as this is a new procedure for TKA, few published studies have reported on the outcomes.
The mini-lateral approach is another new technique for MIS TKA. This involves an incision through the skin and fascia made from the tibial tubercle to the lateral epicondyle lateral to the patella (approximately 8 to 10 cm), as shown in Figure 14-6 . The iliotibial band is also cut, along with the underlying fat pad and capsule. A retractor can be placed underneath the patella to the medial aspect of the tibia and then used to push the patella medially without eversion, releasing soft tissue as necessary. The bone cuts can then be made with the knee in 60 degrees of flexion without dislocating the joint. The bone cuts of this approach can be performed from the lateral side, with lateral-to-medial cutting jigs designed for the lateral approach, as shown in Figure 14-7 . The components can then be placed, followed by closure of the soft tissues.