The introduction of minimally invasive surgical techniques in total joint arthroplasty has ushered in a range of new terminology that often is unclear. Phrases such as “less invasive”, “minimally invasive”, “mini-incision”, “small-incision” and “muscle sparing” are often used. However, those terms lack precise definition. One related term in the realm of knee surgery is quadriceps-sparing total knee arthroplasty (TKA).2,7 A trademark has been applied to one technique for minimally invasive TKA involving a short medial arthrotomy approach that extends to the superior pole of the patella (Quad-Sparing™ TKA, Zimmer, Warsaw, IN).2,7 Because of the anatomic descriptor attached to that trademarked technique, one may infer that a clear anatomic basis exists for the claim of sparing the quadriceps. Several authors have stated the vastus medialis muscle has three distinct patterns of insertion relative to the superior pole of the patella, and additionally suggest most patients have an insertion at or proximal to the superior pole.2,7 However, no data support those contentions. Subsequently, various forms of midvastus, subvastus, and tri-vector retaining approaches for TKA also have been termed quadriceps sparing.
We designed our study to clarify the surgical anatomy of the medial side of the knee in reference to quadriceps- sparing total knee arthroplasty. We specifically sought to determine: (1) the distal most insertion point of the vastus medialis (as measured as a percentage of the patellar length); (2) the angle of insertion of the vastus medialis on the patella (as measured relative to the long axis of the femur); and (3) the length of the tendon of the vastus medialis (as measured from the edge of vastus medialis muscle to the closest point on the patella). With those findings we determined which, if any, TKA approaches could be definitively termed quadriceps sparing based on the surgical anatomy.
MATERIALS AND METHODS
We directly measured the key anatomic landmarks of the extensor mechanism on the medial side of the knee. To determine the insertion point of the VMO on the patella we found the inferior border of the muscle, traced its insertion over to the intersection with the medial border of the patella and expressed that measurement as a percentage of overall patellar height from superior to inferior. To determine the angle of insertion of the VMO we used a goniometer and measured the angle in degrees relative to the long axis of the femur. To determine the length of the tendon of the VMO we found the most lateral portion of the muscle and measured the distance in centimeters to the closest point on the patella. These measurements were done at the time of TKA in 100 knees in 100 patients with degenerative arthritis and the same measurements were done in 45 fresh-frozen cadaveric knees without evidence of degenerative arthritis. We also performed high resolution MRI scans on 5 healthy volunteers to analyze the same anatomic parameters. While the findings in the 100 patients with osteoarthritis are of most interest for the surgeon performing TKA we included the cadaveric knees without arthritis to ensure that our results were not confounded by the presence of degenerative arthritis. Similarly, the data from the high resolution MRI scans in 5 healthy volunteers allowed us to ensure that our results were not confounded by the skin incision and surgical dissection itself.
In 100 consecutive patients (100 knees) having primary total knee arthroplasty, the distal portion of the extensor mechanism on the medial side was exposed to define the inferior border of the vastus medialis obliquus (VMO) muscle and tendon. The point of insertion of the inferior border of the VMO on the medial edge of the patella was noted and recorded as a percentage of the overall patellar length from proximal to distal (such that the superior pole would represent 0% and the inferior pole 100%). The angle of insertion of the VMO on the patella was measured directly with a goniometer referencing the long axis of the femur. The leading edge of the VMO muscle was identified and the length of the tendon of the VMO was defined as the shortest distance between any portion of the patella and the VMO muscle fibers. These measurements also were made in 45 entire cadaveric leg specimens. Additionally, five patients had high-resolution MRI scans of the knee with a 3-Tesla MRI machine, and had the same set of measurements of the medial extensor anatomy. The patients having TKA included 65 women and 35 men with a mean age of 67 years (range, 42-88 years). The cadaveric leg specimens were obtained from 25 males and 20 females with a mean age of 72 years (range, 55-89 years). The five patients who had MRI scans included four men and one woman with a mean age of 33 years (range, 24-44 years).
The medial anatomy of the extensor mechanism was consistent. The inferior edge of the VMO tendon inserted at or near the midpole of the patella in each case. The mean point of insertion of the tendon of the VMO was 50% of the distance from superior to inferior (range, 45-55% of the distance from superior to inferior) (Fig 1, Table 1).
The angle of insertion of the tendon of the VMO showed little variability with a mean insertion angle that measured 50° (range, 46°-52°). The length of the tendon of the VMO showed a bimodal distribution with 100 of the 150 knees (combined patients, cadaveric specimens and normal volunteers) (66%) having a relatively short tendon that measured a mean of 1.2 ± 0.1 cm and 50 of the 150 knees (combined patients, cadaveric specimens and normal volunteers) (33%) having a relatively long tendon that measured a mean of 2.2 ± 0.2 cm in length (Fig 2).
Based on these findings, any surgical approach to the knee that extends more proximal than the midpole of the patella violates a portion of the quadriceps insertion and cannot be considered quadriceps sparing.
We designed this study to clarify the surgical anatomy of the medial side of the knee in reference to so-called quad- riceps-sparing TKA. We determined: (1) the distal most insertion point of the vastus medialis (as measured as a percentage of the patellar length); (2) the angle of insertion of the vastus medialis on the patella (as measured relative to the long axis of the femur); and (3) the length of the tendon of the vastus medialis (as measured from the edge of vastus medialis muscle to the closest point on the patella). Those findings show any capsular incision that extends proximal to the midpole of the patella violates a portion of the quadriceps tendon and cannot be considered quadriceps sparing. The trademarked technique for a quad- riceps-sparing TKA, which involves a medial capsular incision to the superior pole of the patella, fails to meet the anatomic definition of quadriceps sparing.2,7
The limitations of this study focus on the inclusion of only one surgeon's patients with the possibility of selection bias. Most subjects in each of the three parts of this study were Caucasian, therefore this study does not allow us to comment scientifically on possible anatomic differences regarding the musculature around the knee among other ethnic groups. Previous anatomic studies do not support substantial differences in the muscular anatomy around the knee based on ethnicity, but this study cannot support or refute that position.3-6 The study did not address the clinical implications of violating a portion of the quadriceps insertion. Whether incising 5%, 10%, or 25% of the quadriceps insertion has any negative impact on the early or late function of a patient after knee arthroplasty is not addressed by this anatomic study. However, one recent prospective randomized trial showed quicker return of active straight leg-raising and better early range of motion when a mini-subvastus approach (which does not violate the insertion of the quadriceps) was used compared with a Quad-Sparing™ approach.1
Our study showed the vastus medialis obliquus consistently inserted at the midpole of the patella. That finding is in broad agreement with several recent anatomic studies of the medial anatomy of the extensor mechanism of the knee.3-6 Therefore, it is clear any surgical approach to the knee that extends more proximal than the midpole of the patella will detach a portion of the quadriceps tendon from its insertion on the patella. In support of a particular surgical technique, some authors recently suggested it is uncommon for the quadriceps to insert below the superior pole of the patella.2,7 The scientific basis for the authors' contention is unclear because no supporting clinical or basic scientific data have been presented. In contrast, the results of the current study and others show little variation in the location of the insertion point of the quadriceps at the midpole of the patella medially.3-6
The angle of insertion of the vastus medialis obliquus on the patella showed little variation, with a mean of 50° as measured relative to the long axis of the femur. This finding is within the range reported in other cadaveric dissections.3-6 Three relatively recent studies suggest angles of insertion of 52°, 53°, and 57° with a standard deviation of 6 or 7 in each of those studies.3,4,6 One study found knees with a varus mechanical axis had an angle of insertion that was lower (mean 52°) compared to the overall group (mean 57°).6 The current study included 100 patients who were having TKA, and many of those patients had a preoperative varus deformity. Therefore, it is not surprising the mean angle of insertion in this study (50°) was at the lower end of what has been reported previously.
The length of the tendon of the vastus medialis obliquus showed some variability. Most patients had a relatively short tendon that measured just greater than 1 cm at its closest point to the patella. However, a substantial number (50/150 patients, 33%) of the patients had a longer tendon (mean length, 2.2 cm). We could find no other data in the literature specific to the length of the vastus medialis obliquus tendon. From a surgeon's perspective this difference in tendon length may influence the perception of how inferiorly the vastus medialis inserts on the patella (Fig 3). In standard open TKA procedures with a typical medial parapatellar approach it is uncommon to see the entire inferior border of the VMO during the procedure. The leading edge of the VMO often is visualized close to the superior pole of the patella, but the inferior edge is not necessarily seen. This could be akin to seeing only the bow of a ship. To truly identify the inferior border of the VMO requires dissection more distal and more medial than many surgeons may appreciate. When looking through a surgical incision without additional medial dissection to find the inferior border of the VMO, one could easily misidentify the most prominent part of the VMO (the point closest to the patella, akin to the bow of the ship) as the most inferior part of the VMO. Because that most prominent portion often lies close to the superior pole of the patella, some surgeons may then mistakenly presume the VMO inserts at the superior pole of the patella.
As interest and expertise in less invasive and minimally invasive techniques for TKA are developed, precise language to describe surgical technique is valuable. Anatomically based descriptors of technique can be useful to patients and surgeons if the scientific basis for the terminology is clear. In contrast, anatomically based catch phrases that have not been validated by solid scientific inquiry are a disservice. This study clearly shows any medial arthrotomy that extends more proximal than the midpole of the patella detaches a portion of the quadriceps tendon from its insertion on the patella. Therefore, the term quadriceps sparing should not be applied to any surgical approach with a capsular incision that extends more proximal than the midpole of the patella.
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