Total knee arthroplasty (TKA) is historically an extremely successful operation with investigators typically reporting 95% good to excellent results and greater than 94% survival at 10 to 15 year followup in conventional fixed- bearing TKA.3,5,13 Beginning in the late 1990s, there has been a trend toward a more minimally invasive approach to TKA. Although several approaches were initially described, none replaced the standard TKA technique. However, the work of Repicci and Eberle14 and Romanowski and Repicci17 on unicondylar knee replacement encouraged further interest in a limited surgical approach. Their successful techniques provided a foundation for minimally invasive surgery (MIS) total knee replacement.
Several limited approaches have evolved from the traditional extensile approach including the limited medial parapatellar arthrotomy, the limited midvastus approach, the limited subvastus approach, and the quadriceps- sparing approach. These approaches can be easily converted to a traditional extensile approach if necessary.
Several proposed advantages of less invasive surgery include diminished postoperative morbidity, reduced postoperative pain, decreased blood loss, and a quicker recovery. Quicker recovery presumably occurs as a result of less soft tissue trauma including the need for lateral release. Although there are numerous theoretical advantages to MIS TKA, there are limited clinical data on these patients.
We asked whether minimally invasive surgery influenced the need for lateral release.
MATERIALS AND METHODS
We analyzed a consecutive series of 83 patients (106 knees) undergoing TKA performed by a single surgeon (GRS) from September 2, 2003 to June 15, 2004. The knees are divided into two subgroups: a minimal incision group with a limited medial parapatellar arthrotomy and a traditional approach group. The incidence of lateral release of the patella in each subgroup is recorded and the two groups are compared to determine if a minimally invasive approach to total knee arthroplasty affects the incidence of lateral release of the patella.
There were 55 women and 28 men. There were 14 women and nine men having bilateral procedures. The average age of all 83 patients was 65 years (range, 45-84 years). The average height was 65 inches (range, 58-74 inches). The average weight was 190 pounds (range, 108-350 pounds). The average body mass index (BMI) was 31.3 (range, 18.5-53.3). Seventy-eight patients had a diagnosis of osteoarthritis, two patients had rheumatoid arthritis, and three patients had osteonecrosis. There were 57 left knees and 49 right knees. The average length of the incision in extension at the completion of surgery of all 106 knees was 5.4 in (range 3.75-8.25 in). The average preoperative alignment was 1.3° varus. (Table 1) All patients had a standard workup preoperatively and underwent a routine postoperative protocol described previously.4
The average alignment was 1.3° varus (range, 25° valgus to 20° varus). Of the 48 knees with a standard approach, 40 had a preoperative varus deformity (< 5° valgus), three were in neutral preoperatively (5-9° valgus), and five had a preoperative valgus deformity (> 9° valgus) (Table 2). Of the 58 knees treated with a mini approach, 47 were in varus, four were in neutral, and seven were in valgus preoperatively (Table 2).
Forty-eight knees (45%) had a standard incision greater than or equal to 5.5 inches and a standard medial parapatellar arthrotomy. The average length of the incision in extension at the completion of surgery of all 106 knees was 6.3 inches (range, 5.5-8.25 inches) (Table 1). Seventeen women and 20 men received a standard approach. The average age was 64 years (range, 45-78 years). The average height was 67 inches (range, 58-74 inches). The average weight was 202 pounds (range, 108- 350 pounds). The average BMI was 32.1 (range, 18.6-53.3). Thirty-four patients had a diagnosis of osteoarthritis, two patients had rheumatoid arthritis, and one patient had osteonecrosis. There were 26 left knees and 22 right knees. The average preoperative alignment was 1.4° varus (Table 1).
Fifty-eight knees (55%) had a mini approach (an incision less than 5.50 in) and limited medial parapatellar arthrotomy (Table 1). The average length of the incision in extension at the completion of surgery of all 106 knees was 4.7 inches (range, 3.75-5.25 inches) (Table 1). Forty women and eight men had a mini approach. The average age was 67 years (range, 46-84 years). The average height was 65 inches (range, 59-73 inches). The average weight was 180 pounds (range, 114-270 pounds). The average BMI was 30.5 (range, 19.8-46.4). Forty-six patients had a diagnosis of osteoarthritis, no patients had rheumatoid arthritis, and two patients had osteonecrosis. There were 31 left knees and 27 right knees. The average preoperative alignment was 1.2° varus (Table 1).
The initial thickness of the patella in all knees was measured and varied between 15 mm and 29 mm. The thickness of the patella at the end of the procedure measured between 15 mm and 27 mm. The overall thickness of the patella remained the same in 39 knees, although the range of change was from a decrease of 1 mm (15 knees) to an increase of 3 mm (two knees). Sixteen patellas increased in thickness by 2 mm, and 34 increased in thickness by 1 mm.
The technique of mini-incision total knee arthroplasty has been well described.4,19 In this series, we used a limited medial parapatellar arthrotomy or a standard medial parapatellar approach in all 106 knees. Although the length of the initial incision was made as small as possible, it was extended as needed to facilitate the procedure.
The instrumentation for surgery included modified 4-in-1 multireferencing instruments (Zimmer, Warsaw, IN) that facilitated placement within a smaller soft tissue envelope. This is of particular importance because the patella is not everted during the procedure, but is laterally subluxated. Although the instrumentation had been modified to fit in a smaller space, there was no difference in the amount of bone resection of the femur, tibia, or patella.
Tibial resection is accomplished with the assistance of an extramedullary guide. The tibial guide is centered on the tibial tubercle and set at the appropriate depth (approximately 10 mm from the lesser involved compartment) and slope (approximately 7°). The tibia is then cut perpendicular to the mechanical axis while the knee is in 90°of flexion.
After the proximal tibial piece has been removed, there is laxity of the joint in flexion and extension, facilitating placement of the femoral instrumentation. The anteroposterior (AP) axis line (Whiteside's line) is drawn on the uncut surface of the femur and a centering hole is made in the femur just above the inter- condylar notch. An intramedullary rod is set in place that references the most prominent femoral condyle. The distal femur is resected with the appropriate degree of valgus, as determined by preoperative planning. After resection of the distal femur, either the AP axis, which is easier to identify with the limited approach, or the epicondylar axis, is determined. The femur is sized, the AP cutting block is secured in the appropriate degree of external rotation, and the bone cuts are completed.
After the knee is appropriately balanced it is flexed to 90° and the box is cut on the femur to accommodate a posterior stabilized knee. The knee is then hyperflexed, externally rotated, and the tibia is subluxated anteriorly. The tibial surface is sized and the appropriate tray is placed. The tibial tray rotation is referenced to the tibial tubercle, the femoral notch cut, and the malleoli of the ankle.
We prefer to prepare the patella after femoral and tibial re- section because it results in greater potential space in the knee, allowing for easier patellar tilt. The patella is tilted with the knee in full extension or slight flexion. However, the entire extensor mechanism does not have to be twisted or everted. The thickness of the patella is measured. Using the appropriate sized reamer, the patella is resected to the appropriate depth. The three holes for the patellar prosthesis can be made at this time and a trial button placed. The final thickness of the resurfaced patella is measured and compared to the original thickness.
To optimize patellar tracking, a lateral release of the patella is sometimes necessary. Although some surgeons7 advocate releasing the tourniquet prior to tracking assessment, we feel that this is not necessary. Consequently, with the tourniquet inflated and trial implants in place, a towel clip is used to provide gentle tension on the extensor mechanism as the knee is flexed. If the patella is not central, does not track appropriately, or lifts off, a lateral release is performed. The lateral retinacular release is performed with an inside-out technique with preservation of the lateral superior geniculate artery.
The trial implants are removed and the true implants cemented into place. A Legacy Posterior Stabilized (LPS) or LPS Flex Prosthesis (Zimmer) was implanted in all patients. After the cement hardens, a few stitches are placed to begin closure of the medial parapatellar arthrotomy. The knee is flexed again and patellar tracking is assessed again for appropriate alignment. The remainder of the closure is performed in a standard fashion. At the completion of the procedure, the length of the incision is measured with the knee in extension.
Two knees (two patients) had an unresurfaced patella. One patient had patelloplasty in both knees for very thin patellas (15 mm). Of the remaining 102 knees, 23 were implanted with a size 29 patella, 56 with a size 32, 19 with a size 35, and four with a size 38.
Sixteen knees (15.1%) underwent patellar release to achieve proper patellar tracking. Seven of 48 knees receiving a standard approach (14.6%) required patellar release and nine of 58 knees (15.5%) receiving a mini approach required release. Of the seven patients with standard incisions who had patellar releases, five had a varus deformity preoperatively and two were in valgus (Table 3). Of the nine mini approaches requiring patellar releases, three knees were in varus preoperatively, three were in neutral, and three were in valgus (Table 3).
Of the 41 patients with standard incisions who did not have patellar releases, 35 had a varus deformity preoperatively, three were in neutral, and three were in valgus (Table 4). Of the 49 mini approaches not requiring patellar releases, 44 knees were in varus preoperatively, one was in neutral, and four were in valgus (Table 4).
Minimally invasive total knee arthroplasty is a relatively new technique with few published clinical results. However, several advantages to MIS TKA have been described during the initial postoperative period, including improved gains in early flexion.1,6,10,11,20 Additionally, decreased pain scores, reduced analgesia use,10,11 decreased blood loss,1,10,11 improved quadriceps function,1,2,10 and shortened postoperative stay have been described.1,2,10
Appropriate patellofemoral tracking is important to postoperative outcomes after TKA. Lee et al12 discussed optimizing patellofemoral tracking during TKA. Their series consisted of 57 patients who underwent 65 primary TKAs between 1993 and 1994 with a standard technique. There was varus alignment in 42 knees, neutral alignment in six knees, and valgus alignment in 17 knees. The average precut patellar thickness was 23.8 mm and thickness with the button was 21.5 mm. Two knees (3%) required formal lateral release for patellar maltracking. Laskin9 reported his series of 178 patients undergoing TKA and had a lateral release rate of 6%. Our series showed a higher release rate at 15%.
Our data show that there was no difference in patient age, diagnosis, height, or preoperative alignment between those receiving a standard approach and those receiving a mini approach. There was a trend toward lower BMI and weight in those receiving a mini approach, and more women than men had mini approaches. There were similar rates of patellar release in the mini and standard approaches. Overall alignment (whether varus, neutral, or valgus) did not influence the ability to perform a mini approach.
There have been several reported complications associated with lateral releases of the patella including decreased transcutaneous oxygen tension, wound discoloration, and infection.8 Others have shown an association with postoperative patellar fractures15,16 or decreased patellar vascularity18 thought to be attributed to sacrifice of the superior geniculate vessels during the release. Thus, lateral release should be performed only after correct femoral and tibial component rotation and restoration of patellar height has been ensured.12 However, our data show that using the minimally invasive techniques of total knee arthroplasty does not alter the incidence of lateral release of the patella.
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