Knee function after total knee arthroplasty (TKA) is closely related to surgical approach. The standard medial parapatellar approach splits the quadriceps tendon, detaching it and the entire vastus medialis from the upper medial corner of the patella.13 The subvastus approach preserves the extensor mechanism by elevating the inferior border of the vastus medialis from the medial intermuscular septum and adductor aponeurosis5; however, this procedure may be difficult in heavily muscled or obese individuals.11 An alternative to the parapatellar approach was considered in which the quadriceps tendon is left entirely attached to the upper edge of the patella, and the vastus medialis either is separated from the vastus intermedius or split between the vastus medialis obliquus and main body of the vastus medialis (Fig 1). Lieb and Perry6 described the vastus medialis to consist of two portions, the vastus medialis obliquus and the vastus medialis longus. The vastus medialis obliquus is located distally in the vastus medialis muscle and stabilizes the patella.6 The midvastus approach has been advocated by Engh and Parks as an alternative to the standard medial parapatellar approach.3 This approach is beneficial for preserving the medial vector along with the fibers of the vastus medialis muscle, and White et al14 described a clinical advantage with this approach. However, this approach splits the vastus medialis obliquus and risks lacerating the nerves to the vastus medialis and vastus medialis obliquus.
The purpose of the current study was to observe the nerve supply to the distal oblique fibers of the vastus medialis, and to determine how surgical approaches that involve the vastus medialis may damage its nerve supply, especially to the distal portion.
MATERIALS AND METHODS
Ten fresh-frozen normal knee specimens were selected after visual inspection showed no osteoarthritic changes. Three retrieved knee specimens with known clinical history also were obtained with TKA components intact. The retrieved specimens all had replacement by the senior author (LAW) and were implanted with Ortholoc III components (Wright Medical Technology, Arlingon, TN) through the intervastus approach (splitting between the vastus intermedius and vastus medialis) for surgical exposure. No extension lag was apparent at clinical followup, and each was functioning well at the time of the patient’s death. The average time in service was 5.7 years and the cause of death was unrelated to the knee surgery.
Each specimen was thawed overnight in the laboratory and then the skin and subcutaneous tissues were removed the following morning. The nerve supply to the vastus intermedius and vastus medialis was dissected from the level of the femoral head to the gross neuromuscular junction. Next, the nerve fibers were dissected carefully into the body of the muscle and traced to the point where they entered a muscle fascicle or passed through the muscle into the periosteum or capsule of the knee. Special attention was given to nerve fibers in the areas where the vastus intermedius and medialis joined, and where the vastus medialis longus and vastus medialis obliquus blended together, to identify any potential areas where the nerve supply to any of the muscles could be damaged by surgical dissection.
No visual differences in nerve pattern were observed between the normal and retrieved knees. None of the retrieval specimens in which the intervastus approach was used had gross evidence of muscle denervation. The muscle fibers were red and appeared healthy with no evidence of fibrofatty degeneration as generally is seen with denervation. There was no change in thickness or palpable difference in firmness when the distal oblique fibers were compared with those of the main body of the muscle.
Two patterns of distribution, posterior and central, were found. In the posterior pattern, two nerves branched from the femoral nerve to innervate the vastus medialis. The medial branch of the femoral nerve ran along the posterior edge of the vastus medialis, sending branches into the main body of the vastus medialis, and terminated as three or four branches which entered the vastus medialis obliquus fibers of the muscle posteriorly, sending multiple small branches into muscle fascicles and finally terminating in the capsule of the knee. Small nerve fibers crossed between the main body of the vastus medialis and distal oblique portion so that there was no clear watershed or cleavage plane in innervation of the vastus medialis and vastus medialis obliquus. The lateral branch of the nerve ran at the lateral side of the vastus medialis, sending small branches into the vastus medialis muscle fascicles (Fig 2).
In the central pattern of innervation, the small lateral branch did not differ from that found in the posterior pattern (Fig 3). The main innervation was through a large central trunk that entered the main body of the muscle, sending branches throughout the substance of the vastus medialis and distal oblique fibers of the muscle. Two normal specimens and one retrieved specimen had the central pattern of innervation and the remaining 10 specimens had the posterior pattern.
The vastus intermedius was innervated by a separate large branch of the femoral nerve in all specimens. In none of the specimens were nerve fibers found crossing between the vastus intermedius and vastus medialis muscles.
Choice of surgical approach for TKA may influence the postoperative results if it affects quadriceps muscle function. Imbalance of the quadriceps muscle has been reported to cause extensor mechanism failure, and consequently, patellar maltracking. Luo et al8 reported that slight variations in tensile loads produced by the various elements of the quadriceps mechanism could cause marked patellar tracking abnormalities. Bindelglass et al1 reported a high rate of patellar tilt and maltracking, and they suggested that maltracking may result because static, intraoperative testing cannot always predict the dynamics of quadriceps function and the effect on patellar tracking. Bose et al2 and Lieb and Perry7 reported the important role of the vastus medialis obliquus in stabilizing the patella. Maintaining innervation of the vastus medialis is likely to affect quadriceps function, and therefore is an important part of the surgical technique in TKA.
To maintain proper function of the vastus medialis muscle, some authors have recommended leaving undisturbed all or part of the attachments of the vastus medialis to the upper medial edge of the patella.9 However, the subvastus approach often is difficult in muscular or obese patients, and an alternative approach that maintains attachment of the quadriceps tendon and medial muscle force to the upper medial corner of the patella but also allows easy access to the knee would be attractive. The midvastus approach, which separates the distal oblique portion from the proximal portion of the vastus medialis, leaving the proximal portion attached to the upper medial corner of the patella, is designed to achieve this effect, but innervation of the distal portion of the vastus medialis is a matter of concern.10 This current study suggests that a surgical approach that splits the midportion of the vastus medialis obliquus is likely to damage the innervation of the muscle. As reported by Thiranagama12 and Günal et al,4 innervation of the vastus medialis obliquus is not entirely separate from that of the rest of the vastus medialis. The largest nerve trunks to the vastus medialis obliquus traverse the vastus medialis in approximately ⅓ of the cases.
Although the number of specimens in our study was small, there were enough specimens to show a tendency of the nerve distribution. It is clear that innervation to the vastus medialis obliquus and the main body of the vastus medialis are compromised by attempting to dissect between the two structures. In ⅓ of the cases as shown by our study and by that of Thiranagama,12 major nerve trunks are transected as the muscle fibers are split. Even in cases with the more common posterior entry of nerve branches into the vastus medialis, splitting the muscle between fascicles would damage any nerves lying in these intervals. This explains the high frequency of partial denervation of the vastus medialis obliquus and vastus medialis reported by Parentis et al10 using the midvastus approach. The approach that extends proximally from the upper medial corner of the patella and divides the vastus medialis and vastus intermedius offers the advantage of maintaining tendon and muscle attachment to the proximal medial corner of the patella and therefore maintaining the medial vector while avoiding damage to the innervation of the vastus medialis obliquus. This approach is extensile, and can be extended proximally to expose the midshaft of the femur if necessary.
We appreciate the editorial assistance of Diane Morton, MS, in preparation of this manuscript, also William Andrea, CMI, for preparation of the illustrations.
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