Surgical Exposure in Total Knee Arthroplasty

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CHAPTER SYNOPSIS:

Adequate surgical exposure is one of the keys to successful total knee arthroplasty. The approach selected is dependent on surgeon knowledge and experience, individual patient anatomy, and location of any prior surgical incision(s). Revision total knee arthroplasty presents certain challenges to the exposure and may require additional procedures such as tibial tubercle osteotomy and the use of vascularized soft tissue flaps for adequate coverage.

IMPORTANT POINTS:

1

The skin incision
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  Straight anterior
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  Prior surgical incisions
2

Deeper dissection
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  Medial parapatellar
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  Subvastus
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  Mid-vastus
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  Intervastus
3

Additional exposure
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  Soft tissue release
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  Quadriceps turndown, or V-Y plasty
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  Tibial tubercle osteotomy
4

Closure
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  Primary
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  Vascularized flaps

CLINICAL/SURGICAL PEARLS:

1

Patient selection
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  Consider factors such as prior surgery, obesity, skin status, and muscle mass when choosing the approach
2

Skin incision
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  Straight anterior is most common.
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  Use prior incision when possible.
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  Delay arthroplasty if adequate soft tissue coverage cannot be obtained.
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  Avoid the use of retractors after entering the knee.
3

Deep dissection
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  Avoid damage to the vastus medialis and its neurovascular supply.
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  Use stepwise fashion of soft tissue release (osteophytes, fat pad, anterior quadriceps expansion, lateral patellar retinaculum).
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  Use quadriceps turndown, or V-Y plasty in cases of quadriceps contracture.
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  Tibial tubercle osteotomy is best when preliminary exposure is through the subvastus, mid-vastus, or intervastus approach.
4

Closure
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  Use augmented soft tissue closure as needed.
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  The medial gastrocnemius muscle belly is an effective vascularized flap.

CLINICAL/SURGICAL PITFALLS:

1

Visualization and exposure
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  Inadequate incision to allow visualization
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  Patient factors such as obesity or contracture
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  Avoid excessive stripping of capsule and ligaments, which can devascularize the bone and affect stability.
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  Coonse Adams turndown can damage the extensor mechanism and lead to patellar avascular necrosis.
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  Use oblique saw cuts during tibial tubercle osteotomy and use care in fixation to avoid a stress riser, which can result in fracture.
2

Patella maltracking
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  Minimize vastus medialis muscle damage.
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  Use caution to avoid neurovascular injury during approach.
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  Perform adequate patella retinacular release.

VIDEO AVAILABLE:

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Surgical exposure in revision total knee arthroplasty

HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM

Exposure is the cornerstone to successful total knee arthroplasty (TKA) and is accomplished by the application of knowledge and experience with the anatomy and function of the soft tissues in the lower extremity. Often the initial incision is inadequate and must be lengthened to achieve adequate exposure, so all exposures made should be extendable. One of the worst mistakes in TKA is to attempt to do the operation through a small incision.

The skin receives most of its blood supply from the femoral artery and innervation from the femoral nerve, so the vessels and nerves pass from medially to laterally. Therefore, laterally placed skin incisions are more likely to have well-vascularized and innervated skin edges on both sides. However, arthrotomy usually is done medial to the patella, so skin incisions lateral to the patella would need to be excessively long to allow adequate access to the joint. The best compromise is a straight anterior incision in primary TKA ( Fig. 17-1 ). It can be extended proximally and distally to gain access to the shafts of the femur and tibia and even farther distally to gain access to the medial gastrocnemius muscle. Consideration for blood supply and management of the subcutaneous fat are important issues in the obese lower extremity. Generally, extensive subcutaneous dissection is not good practice in exposure for TKA, but in cases of obesity in which the thick layer of subcutaneous fat is dense and fibrotic, some undermining of the subcutaneous tissue is necessary to displace the patella laterally without excessive pressure and retraction on the subcutaneous fat layer itself. Blood supply to the subcutaneous fat layer comes through the subdermal plexus and the deep plexus that lies just superficial to the deep fascial layer, so the method of dissection of the subcutaneous fat should avoid dissecting directly into the fat layer. In these cases, dissection in the subfascial plane is the safest way to loosen the subcutaneous fat from the patella and anterior capsular structures ( Fig. 17-2 ).

The skin incision should be at least as long as the deep incision or the surgical team risks stretching the skin and subcutaneous fat so extensively that necrosis of the fat and skin occurs, often leading to catastrophic infection. Retraction while transecting the skin and subcutaneous tissue should be very gentle, and after entry into the knee through the quadriceps mechanism and capsule, retractors no longer should be applied to the skin and subcutaneous tissue.

In cases with multiple surgical scars from previous operations, a good rule of thumb is to reenter the knee through the incision most recently done, the most laterally placed, the longest, and the least adherent to deep fascial tissues. When one incision meets all these criteria, it is clearly the correct incision to access the knee if it is in reasonable position. In rare cases where a clearly effective and safe entry into the knee is not apparent, the skin incision can be made prior to inflating the tourniquet, the skin edges can be inspected, and if poor vascularity is present, the skin should be closed and an inflatable skin expander placed in the subfascial place next to the incision. The arthroplasty should be postponed until adequate soft tissue coverage can be achieved. In cases where this is not applicable such as an infection with loss of skin and capsular tissue or patellar tendon, the procedure can be continued and then at the end of the operation, a medial gastrocnemius flap can be used to close the capsular defect, leaving a surface that will require a split-thickness skin graft after deep closure has been done.

Choice of surgical approach for total knee replacement may have a major influence on postoperative results because of its effect on the quadriceps muscle function. Imbalance of the quadriceps muscle can cause patellar maltracking and complete failure of the quadriceps mechanism. Slight variations in tensile load produced by the various elements of the quadriceps mechanism could cause marked patellar tracking abnormalities. High rates of patellar tilt and maltracking have been reported, and the authors have suggested that this may result from dynamic imbalance in the quadriceps muscle group that is difficult to detect at the time of surgery. The vastus medialis and vastus medialis obliquus are especially important stabilizers of the patella, and even mild neurologic damage to these structures is likely to result in patellar tracking abnormalities that would be difficult to detect intraoperatively but would significantly compromise later results.

The standard medial parapatellar approach splits the quadriceps tendon, detaching the medial half of the quadriceps tendon and the entire vastus medialis from the medial half of the patella. This is an effective approach to the knee but risks patellar tilt and subluxation in a significant number of cases. An approach to the knee that does not split the quadriceps mechanism, the subvastus approach, preserves the attachments of the vastus medialis to the patella, but it does elevate the vastus medialis from the medial intermuscular septum and adductor aponeurosis. This procedure is difficult in heavily muscled or obese patients, often requiring tibial tubercle osteotomy, and also may damage the vastus medialis because of heavy retraction and stretching. Approaches to the knee that split muscle fibers within the vastus medialis (trivector and mid-vastus approaches) offer more convenient exposure than does the subvastus approach, and also leave muscle attachments to the superior medial border of the patella, thus facilitating patellar stability ( Fig 17-3 ). Both of these approaches risk damage to the nerve and blood supply to the vastus medialis obliquus and also may damage the muscles directly by causing excessive retraction and pressure on the muscle tissues. Although the vastus medialis commonly is described as consisting of two portions, the vastus medialis obliquus and the vastus medialis longus, the two cannot be separated from one another anatomically, and the nerves and arterial supply to the muscle that run in the intervals between the muscle fascicles are at risk of damage with any approach that enters the vastus medialis muscle even when splitting the muscle in line with its fibers. Electromyographic evidence of partial denervation of the vastus medialis obliquus and even of the proximal edge of the split vastus medialis muscle tissue has been reported after using the mid-vastus approach to the knee. Anatomic studies of the nerve supply to the vastus medialis consistently show that the main nerve supply enters posteriorly and sends branches that pass between the fascicles in the junction between the vastus medialis and intermedius, so these nerve branches are at risk in any approach that splits the vastus medialis muscle. Careful anatomic evaluations of the nerve supply to the vastus medialis done on large numbers of cadaver specimens show that 15% to 20% of specimens have large neural trunks within the substance of the vastus medialis that send branches directly into the vastus medialis obliquus fibers ( Figs. 17-4 and 17-5 ). These anatomic findings suggest that splitting the vastus medialis obliquus from the main vastus medialis likely will do minor damage in most cases and severe damage in the 20% in which the main nerve to the vastus medialis obliquus enters through the center of the muscle. The vastus intermedius and rectus femoris muscles are supplied by separate nerves that come off the femoral nerve high and go directly into the muscle tissue, ending just above the knee. Therefore, the interval between the vastus medialis and the vastus intermedius virtually is devoid of nerve supply and can be dissected without damaging nerve supply to either the vastus medialis or the conjoined rectus femoris and vastus intermedius.

Another advantage of the intervastus approach, splitting the interval between the vastus medialis and intermedius, is its extensibility. The incision can be extended farther proximally to expose the distal two-thirds of the femoral shaft without endangering nerve or blood supply to any of the muscles in the thigh, and it is far from the superficial femoral artery until dissection has been extended to the upper third of the femur.

Entry into the stiff knee or the knee that has massive osteophytes and ligamentous and quadriceps contractures requires extra work other than extensile exposure through the intervastus interval. Whereas stripping the capsule and ligaments from the femur and tibia to facilitate exposure might be tempting, this type of approach often devascularizes massive amounts of the bone structure, and also separates ligaments that will be crucial for stability later. These knees should be exposed in a stepwise fashion. Removal of osteophytes, release of the fat pad from the anterior surface of the tibia, release of the anterior third of the quadriceps expansion into the medial tibial flare, and partial release of the lateral patellar retinaculum often relax the knee enough to allow the patella to be slipped laterally and the knee to be flexed to allow exposure.

The patellar retinacular release is done by dissecting over the top of the patella and exposing the lateral patellar retinaculum. The retinaculum either can be pie-crusted, leaving the deep synovial tissue intact, or released longitudinally, dissecting the intracapsular vessels away from the capsular tissue and stretching the synovial membrane. This can be done leaving the knee joint closed so that a lateral hematoma connecting with the knee joint does not form.

In revision total knee replacement, the patellar component can be removed without everting the patella by using an oscillating saw to cut through the bone–cement interface. Also the tibial polyethylene component can be removed to produce more slack in the capsule and quadriceps mechanism, allowing the quadriceps and capsule to be retracted enough to enter the knee safely. Even after all of these maneuvers have been accomplished, some knees still cannot be exposed adequately without elongating or detaching the quadriceps mechanism. An attempt to stretch the quadriceps often achieves exposure after all of the efforts to loosen the knee have failed. The knee is bent to full flexion and held in position for approximately 1 minute, then flexed a little more and held again, flexed again, progressively continuing to flex the knee to allow the quadriceps mechanism to stretch for 10 to 15 minutes. This stretching maneuver should be done only after all the other conservative releases have been done. It should be emphasized that the ligaments have not been violated nor has the quadriceps mechanism been transected.

Exposure in the presence of quadriceps contracture and fibrosis of the capsule and patellar tendon often requires additional surgical procedure as well as more subfascial exposure. In the few cases remaining that cannot be exposed without detaching the quadriceps from the knee, the quadriceps should be approached. The classic exposure for extensive intra-articular work on the knee is the quadriceps turndown, or V-Y plasty. In this procedure, the quadriceps incision is extended from proximally with a lateral downward incision across the top of the patella and through the lateral retinaculum to the joint ( Fig. 17-6 ). The patella and patellar tendon then become a distally based flap ( Fig. 17-7 ). Excellent exposure can be achieved with this procedure, but extension lag almost always is reported in series that report results of the turndown or classic Coonse-Adams approach to the knee. Not only does the Coonse-Adams turndown damage the function of the quadriceps muscle, it also commonly results in avascular necrosis of the patella with catastrophic consequences.