CHAPTER SYNOPSIS
Disruption of the extensor mechanism is the principal source of complications following total knee arthroplasty. Consequently, substantial attention has been paid to the various surgical approaches, the chief differences between which involve handing of the quadriceps muscle, patella, and patellar ligament. There are five commonly used contemporary approaches for primary total knee arthroplasty: the medial parapatellar, subvastus, midvastus, trivector, and lateral parapatellar approach. The medial parapatellar is the historical gold standard and most commonly used approach. Recent clinical outcomes suggest that the midvastus and subvastus approaches have some advantage with regard to functional recovery of muscle strength and postoperative pain. The midvastus approach is more versatile than the subvastus approach, being less hindered by the bulk of the vastus medialis in large patients, and may result in less postoperative pain after primary total knee arthroplasty.
HISTORY/INTRODUCTION/SCOPE OF THE PROBLEM
Numerous surgical approaches to the knee have been described for total knee arthroplasty (TKA). Each of these approaches typically provides wide exposure of the entire knee joint; however, the surgical approach has been implicated as a cause for several complications of primary knee arthroplasty, and this has led to considerable controversy as to the most appropriate technique of operative exposure. In this chapter, we first review the relevant anatomy, followed by a discussion of the historical development of operative approaches to the knee. We then present five contemporary surgical approaches: the medial parapatellar, the subvastus, the midvastus, the trivector, and the lateral parapatellar approach.
Anatomy
“ The operating surgeon should be so familiar with the anatomy that he can plan his own approach, basing it upon the anatomical accessibility of the lesion ” Abbott and Carpenter, 1945
Vascular Supply to the Knee
Arterial blood supply about the anterior knee is derived from a complex and rich anastomotic network ( Fig. 7-1 ). In considering surgical approaches to the knee, the supply to the anterior skin and patella are of paramount importance, as these two structures reside in the watershed area of vascular supply are most at risk for devascularization.
The integument of the anterior knee receives supply from a fascial plexus that is immediately superficial to the deep fascia. This hexagonal plexus is formed both by medial and lateral vessels, but the medial supply is substantially more dominant than the lateral. The medial vessels are derived from the descending (supreme) genicular artery, the saphenous branch of the descending genicular artery, and an anterior genicular branch of the femoral artery. The lateral side of the plexus is supplied by the superior and inferior lateral genicular arteries.
The saphenous branch of the descending (supreme) genicular artery is the single largest artery supplying the fascial plexus. From its origin, it travels with the saphenous nerve between the sartorius and gracilis muscles and terminates at its anastomosis with the medial inferior genicular artery. Medial incisions typically interrupt the medial blood supply, but the subdermal vascular plexus is well developed and skin necrosis is rarely seen. Lateral incisions can be more problematic; with a dominant medial circulation and a watershed region over the anterior aspect of the knee, occasionally skin necrosis will develop on the lateral side of a laterally placed incision. Most important, in patients who have prior longitudinal incisions about the knee, the most lateral incision that is useful for the procedure should be used in an attempt to preserve the dominant medial blood supply to the skin.
The osseous supply to the patella is also derived from a complex vascular plexus. Scapinelli originally described an extraosseous anastomotic ring that preferentially perforated the patella inferiorly. Subsequent work by Bonutti et al. demonstrated that the blood supply to the patella was based on a loose functional network composed of all the geniculate arteries, including the superior (supreme), superior and inferior medial, superior and inferior, and anterior recurrent tibial. The standard medial parapatellar approach typically results in sacrifice of the supreme genicular and superior and inferior medial genicular arteries. Within the patella, there is also an intraosseous anastomotic network gaining access via the anterior cortical foramen and concentrated in the central third of the patella. Consequently, any disruption of the central third of the patellar osseous structure risks disruption of the entire vascular supply to the bone.
Finally, the popliteal artery, which gives origin to all of the geniculate vessels proximal to the trifurcation, is located in the posterior aspect of the knee. It lies lateral to the midline of the tibial plateau, typically directly posterior to the lateral tibial spine. In this location, it is vulnerable to injury by an errant drill bit, screw, or saw; this risk is mitigated by flexion of the knee when working on the posterior tibial plateau as well as judicious respect of the popliteus muscle, which shields the popliteal vessel from an anterior approach.
Nervous Supply to the Knee
There are four cutaneous nerves about the anterior aspect of the knee: the medial and intermediate cutaneous nerves of the thigh, the infrapatellar branch of the saphenous nerve, and the lateral cutaneous nerve of the calf ( Fig. 7-2 ). The infrapatellar branch of the saphenous nerve (IPBSN) is the most commonly damaged nerve during the operative exposure.
The saphenous nerve arises from the femoral nerve. It arises after the femoral nerve emerges from the adductor canal between the gracilis and semitendinous muscles. The IPBSN branches immediately thereafter as a purely sensory nerve, emerging from the fascia lata medial to and at the level of the inferior pole of the patella with the knee in extension. The IPBSN then divides into superior and inferior branches, which overlap innervation with the lateral cutaneous nerve of the thigh, resulting in a patellar nervous plexus. Like the vascular plexus, because the nervous plexus is dominated by innervation from the medial side of the knee, when transected during the operative approach there is typically hypoesthesia lateral to the incision about the inferior aspect of the knee.
While not directly responsible for sensory innervation of the skin about the knee, the peroneal nerve deserves the constant respect of the surgeon. Tethered at the level of the fibular neck, it is most commonly in jeopardy secondary to elongation and stretch after correction of a valgus deformity of the knee. Direct exploration is rarely indicated, but gentle flexion of the knee and avoidance of circumferential compressive dressings alleviate some pressure on the nerve at this level. When injured, recovery from a complete peroneal palsy is rarely functional, but an incomplete injury more typically regains motor function sufficient to eliminate the need for a brace.
Historical Approaches
Abbott and Carpenter reviewed numerous surgical approaches to the knee in 1945, including 12 to the anterior aspect of the joint. These included both medial and lateral parapatellar incisions, patella-dividing incisions, and quadriceps tendon and patella ligament-splitting incisions ( Fig. 7-3 ). Sir Robert Jones advocated for the patella-splitting approach for complete exposure. However, this approach was largely abandoned in favor of the assorted parapatellar incisions ( Fig. 7-4 ) due to damage to the articular surface of the patella.
Contemporary Approaches
Medial Parapatellar Approach
The medial parapatellar approach was originally described by von Langenbeck in 1878; the name describes the location of the arthrotomy , and the medial side is the most popular approach for total knee arthroplasty ( Fig. 7-5 ). The skin incision may be curved laterally or medially adjacent to the patella or straight up the midline; most commonly, a straight midline incision is chosen. The incision is positioned over the medial third of the patella, and extends approximately 8 cm proximal to the superior pole and 2 cm distal to the tibial tubercle.
Sharp and blunt dissection are carried down to the extensor mechanism, affording visualization of the quadriceps tendon, patella, and patellar ligament. A curvilinear arthrotomy is begun proximally within the medial third of the quadriceps tendon, extending along the medial border of the patella (leaving a cuff of soft tissue for later repair), and concluding along the medial side of the patellar ligament and tibial tubercle.
Alternatively, Insall argued in favor of a straight medial retinacular incision in order to avoid disruption of the vastus medialis insertion into the patella. He began with a lateral parapatellar incision, followed by medial dissection to exposure of the extensor mechanism. The quadriceps tendon was split 8 cm above the patella, extending distally in the midline over the patella, and through the patella ligament to the tibial tubercle. Using sharp dissection, he carefully separated the quadriceps expansion directly from the medial half of the patella along with the synovium, dividing the infrapatellar fat pad to dislocate the patella laterally.
Subvastus Approach
The subvastus surgical approach is purported to be a “more anatomic approach” than the medial parapatellar arthrotomy, insofar as the extensor mechanism is preserved. Theoretically, this improves intraoperative evaluation of patella tracking following component placement and virtually eliminates the risk of postoperative vastus medialis obliquus disassociation and dehiscence. Other considered advantages include decreased need for lateral release and diminished postoperative pain. Likely limitations include difficult exposure in patients weighing more than 200 pounds and in revision surgery.
Following a standard midline incision, a medial skin flap is developed ( Fig. 7-6 ). The superficial fascia is identified proximally and incised in line with the skin incision, curving medially at the level of the patella to avoid injury to the vascular plexus. Using blunt dissection, the fascia is elevated off the vastus medialis down to its insertion until the lower border of the muscle is visualized. A transverse incision is then made through the tendinous insertion of the vastus medialis into the medial capsule, the trailing inferior edge of the muscle is mobilized, and the extensor mechanism is retracted anteriorly and laterally. The arthrotomy is then completed in standard fashion medial to the patellar ligament to the tibial tubercle. The patella is everted with the knee in extension; as the knee is slowly flexed, the vastus medialis is bluntly dissected from the intermuscular septum to complete the exposure.
Related posts:
Soft Tissue Balancing during Total Knee Arthroplasty
Addressing Ligament Deficiency in Revision Total Knee Arthroplasty
Removal of Well-Fixed Components during Revision Total Knee Arthroplasty
Extensor Mechanism Rupture
Adjunctive Fixation in Total Knee Arthroplasty Revision: Stems and Sleeves
Surgical Exposure in Total Knee Arthroplasty
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