Minimally invasive total knee arthroplasty (TKA) has gained popularity over the past several years. We previously described our first year of experience using the mini midvastus approach (MMV).8 We found more rapid recovery compared with the standard medial parapatellar approach, with improved range of motion (ROM) and function at 6 weeks, 12 weeks, and 1 year postoperatively without an increased rate of complications or radiographic outliers.8
Although total knee replacement has proven be a successful procedure, the recovery period is often long and difficult. In 2001 we began performing minimally invasive (MIS) TKA through a modified midvastus approach without patella eversion (MMV approach). During the first year, lower profile, left- and right-sided instruments were developed to facilitate the technique (Fig 1). Our initial study was limited in selection and number of patients and length of followup. This study includes a much larger cohort of patients with longer followup.
Our hypothesis is patients who undergo MIS TKA with a mini midvastus approach will achieve greater early ROM than historically reported. A second hypothesis is postoperative ROM is related to preoperative ROM and body mass index (BMI).
MATERIALS AND METHODS
The study design is a retrospective cohort analysis of patients who underwent MIS TKA through a mini midvastus approach and compared to a previously published control group.8 We reviewed 335 patients who underwent 391 MIS TKAs from September 2001 to September 2004. The first 40 MIS TKAs were previously reported in our earlier series.8 Power analysis indicated with an alpha of 0.05 and power of 0.8, a minimum of 35 patients would be required for this study to discern a difference in ROM of greater than 10°. We considered patients candidates for MIS TKA using the MMV approach if they had greater than 80° of motion and flexion contracture less than 20°. We excluded patients with history of prior open surgery other than isolated meniscectomy and those with poor quality skin. Patients were judged to have poor quality skin if examination showed dermal atrophy, multiple prior scars, or had a history of chronic steroid use or skin ulcers. Diabetes alone was not considered an exclusion criterion. Muscular males greater than 100 kg were generally excluded. There was no exclusion based on deformity or coronal stability. The mean age was 68 years (range, 33-89 years). There were 248 females and 87 males. This represented 62% of patients undergoing TKA during the study period. The mean height and weight were 165 cm (range, 142-193 cm) and 80 kg (range, 45-130 kg), respectively (Table 1). The minimum followup was 3 months and patients were followed up to 3 years. Seventeen patients were lost to followup.
With the leg fully extended, a longitudinal incision was made over the anterior aspect of the knee along the medial border of the patella. The incisions ranged from 8.5 to 12.5 cm with a mean of 10.6 cm. Patients with greater BMI tended to have larger incision. A medial arthrotomy was then performed from the proximal border of the patella to approximately 5 mm medial to the tibial tubercle. The suprapatellar pouch was identified, separated from the underside of the quadriceps tendon, and preserved. The distal extent of the vastus medialis obliquus was identified at the superomedial corner of the patella. The fascia of this muscle was incised obliquely along the line of the muscle fibers for approximately 1 cm. The muscle fibers were then bluntly spread by hand (Fig 2). The total amount of muscle splitting at the completion of the case was generally 1 to 3 cm.
The knee was then flexed approximately 70° to 90°. The patella was subluxated, but not everted. Hyperflexion was avoided except for tibial preparation and component insertion. A Genesis II® (Smith and Nephew, Memphis, TN) posterior-stabilized TKA was used in all patients. The anteroposterior (AP) axis as described by Whiteside and Arima17 was used to align component rotation. We used instrumentation designed for the minimally invasive technique with separate guides for the left and right knee (Fig 3). The same anterior referencing instrumentation was used during the study period. Patella preparation was done with the leg in extension after femoral and tibial bone cuts, when the extensor mechanism is most lax. The patella was rotated approximately 90°, measured, and cut from the medial to lateral direction. All components were cemented using high viscosity Palacos® (Biomet, Warsaw, IN) bone cement. A pneumatic tourniquet was applied to the upper thigh and was generally inflated from 250 to 300 mm/Hg. The tourniquet was released after the cement set to obtain hemostasis. The arthrotomy was closed with interrupted absorbable sutures. The primary differences between the standard and minimally invasive approaches were a smaller arthrotomy without cutting of the quadriceps tendon, preservation of most of the suprapatella pouch, avoidance of patella eversion with less splitting of the muscle, and a smaller skin incision.
All surgeries were performed by one surgeon (SBH) with epidural anesthesia supplemented by femoral nerve block with bupivacaine. The epidural anesthesia was generally kept in place for 48 hours postoperatively. Continuous passive motion was started in the recovery room. Patients were allowed full weight-bearing the day after surgery, first with a walker and then a cane. Postoperative warfarin and pneumatic foot pumps were used for deep venous thrombosis prophylaxis.
Intraoperative data included implant type, need for lateral release, incision size, amount of midvastus split, tourniquet time, and any intraoperative complications. Preoperative and postoperative data included age, gender, weight, diagnosis, and Knee Society scores9 including pain, stability and ROM. Any postoperative complications were recorded. Data on these patients were collected preoperatively and after surgery at 6 weeks, 3 months, and yearly thereafter. Range of motion measurements were performed by one evaluator (SBH) in a standardized fashion using a goniometer. Maximum passive flexion was measured with the patient in the supine position.
All data were entered on a tablet-based PC database. A Student's t test was used to evaluate significance, with p < 0.05 considered significant.
Using the mini midvastus approach patients were able to regain motion faster than we had previously seen with the standard medial parapatellar approach (Table 2).8
Mean ROM using the MMV approach was 111° at 6 weeks postoperatively. This improved to 121° at 3 months postoperatively, 125° at 1 year postoperatively, and 125° at 2 years postoperatively (Fig 4). No patient had an extensor lag. Thirteen knees (0.3%) had a flexion contracture. One knee developed a flexion contracture of 10° and the remaining 12 knees were less than or equal to 5°.
Patients with greater preoperative ROM had greater (p < 0.05) postoperative ROM. Improvement in ROM at 3 months, 1 year, and 2 years postoperatively occurred in all groups based on preoperative ROM (Table 3). Patients with less than 110° of preoperative ROM showed greater (p < 0.05) improvements at 6 weeks and 3 months postoperatively. Only 2% of patients had less than 90° of pre-operative ROM, and the most common ROM was from 100° to 129°. Patients with lower BMI had greater ROM. Patients with a BMI less than 30 had 3° to 4° greater (p < 0.05) ROM at all postoperative intervals. Only 4% of patients had a BMI of greater than 40 with 63% having a BMI of less than 30. Seventy-three percent of the study TKAs were performed in women compared to 65% females for all TKA procedures during the same time. At last followup we observed no difference in postoperative range of motion between men and women.
The tourniquet time for procedures performed during the first year was 63 minutes (September 2001-September 2002) compared with 48 minutes during the last year (September 2003-September 2004). We performed lateral retinacular release in seven knees (2%). Postoperatively, no patient had coronal or AP instability. The mean Knee Society scores at 3 months, 1 year, and 2 years postoperatively were 95, 96, and 95, respectively.
Two patients (0.5%) had deep infections that were successfully treated with two-stage reimplantation procedures. There were no aseptic failures. One knee (0.25%) underwent irrigation and débridement for sterile wound drainage. One patient (0.25%) had superficial wound necrosis that resolved with dressing changes. This patient had a previous open medial meniscectomy incision that was incorporated into the new TKA incision. Her BMI was 27. Eight patients (2%) underwent manipulation under anesthesia.
There has been great interest in using minimally invasive techniques for TKA. The number of surgeons trained to perform this procedure has substantially increased over the past 3 years. We previously published a controlled study comparing a standard medial patella approach with the mini midvastus MIS TKA.6 The approach improved ROM at 6 weeks and 3 months postoperatively, and the technique was not associated with an increased complication rate.6 Our current study included a much larger group of patients on whom the same technique was performed, but with results similar to our initial series.6
We note several limitations. The study is retrospective and does not have a concurrent control group. This could lead to selection bias in the groups, rendering them difficult to compare. We did not include a radiographic analysis, therefore the potential for early loosening or other radiographic complications could not be assessed. This should not influence assessment of ROM.
When compared with our historical control subjects with the medial parapatellar approach, patients undergoing MIS TKA regained motion faster and had greater ROM at 1 and 2 years. In our current study we analyzed various factors influencing ROM. In contrast to the study by Laskin,12 that showed a trend toward great ROM in females, we did not find a relationship between gender and postoperative ROM. Postoperative ROM was related to preoperative motion; however, important gains in ROM were noted in patients with 100° to 120° of motion. Body mass index was correlated with postoperative ROM; however, improvements in ROM occurred in all groups. Ninety-six percent of patients had a BMI of less than 40. Given the small number of patients who had a BMI greater than 40, we do not believe our results can be generalized to this population group.
The overall complication rate was low; there were no fractures, tendon injuries, or neurovascular complications. The infection rate was 0.5%. Although we saw improvements in ROM throughout all groups of patients, we still performed manipulation in 2% of patients. While MIS TKA provided good or excellent ROM in most patients, there were some who could not achieve early ROM without manipulation.
We used instruments specially designed for MIS TKA, which facilitated good exposure without undue stress on the soft tissue. We believe MIS TKA should not be performed with instruments designed for larger exposures. Using larger instruments can lead to excess retraction or soft tissue impingement and malalignment.4 The concept of MIS TKA is not to excessively retract a small exposure to fit large instruments into the knee, but rather to use lower profile, more anatomically-shaped instruments that will easily fit through a less invasive exposure.
Several approaches to MIS TKA have been described.1,2,8,12,14,15 All of these approaches have shown similar clinical results, with a more rapid recovery and improved ROM in the early postoperative period.1,2,8,12,14,15 As with any new technique, there may be an associated learning curve. Because the mini midvastus approach is based on a previously described and commonly performed midvastus technique,3,5-7,10,11,13,16 we believe the learning curve is short. Studies evaluating the early experience with this technique did not show an increase in complications or radiographic outliers.8,12 Boerger et al2 and Tria and Coon15 reported increased tourniquet times together with several complications, making us believe the learning curve may be more difficult with the quadriceps-sparing or subvastus approaches to MIS TKA.
Our results suggest MIS TKA performed with a MMV approach is associated with rapid functional recovery and improvements in ROM. This procedure can be performed safely and provides excellent clinical results.
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