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Minimally Invasive Surgery in Total Knee Arthroplasty: The Learning Curve

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Abstract

The purpose of this study is to quantitate the number of surgeries required to become proficient in minimally invasive surgery total knee arthroplasty (TKA). Operative duration is recorded for the first 100 patients having minimally invasive TKA by a single surgeon (J.H.L.). Learning is graphically represented by plotting time by patient number, generating logarithmic and linear curves that represent the rate of decrease in time (learning).The operative duration of the first block of 10 patients was significantly longer than that of the remaining 9 blocks. The learning curve for minimally invasive TKA approximates 10 patients, and decreases as surgical experience is gained.

This study investigates how many surgeries are required for a surgeon to become proficient in performing minimally invasive total knee arthroplasty (TKA).

The advantages of minimally invasive TKA include reduced hospital stay, decreased postoperative pain medication requirements, faster recovery, and return to function with decreased need for rehabilitation. The success of this method is well established.^1-6

However, a disadvantage of minimally invasive TKA may be its technical challenge. Developing the techniques of minimally invasive TKA is reported to be difficult and time-consuming or requiring extra effort.⁷ Minimally invasive TKA is reported to have a learning curve,^5,8,9 although it has not been quantified.

The investigators report the learning curve for minimally invasive TKA and hypothesize that as experience is gained, learning can be quantitatively demonstrated by a significant decrease in operative time.

Materials and Methods

The investigators prospectively analyzed 100 consecutive patients having minimally invasive TKA performed by a single surgeon (J.H.L.) experienced in standard, primary TKA beginning with his first minimally invasive TKA surgery. All procedures were performed with the assistance of an orthopedic fellow or a physician assistant.

The investigator’s surgical technique has been reported previously in detail.¹⁰ In summary, the approach is mini-midvastus with lateral patellar subluxation without patellar eversion. The hybrid approach involves distal femoral cut first, followed by the tibial cut, and finally by the completion of femoral preparation. The posterior cruciate ligament (PCL) and the patella are spared, and a Genesis II minimally invasive TKA instrumentation and Genesis II implants (Smith & Nephew; Memphis, Tenn) are used. A tourniquet is inflated before skin incision and released after the surgical wound is closed, and a dressing is applied.

Evaluation

The investigators measure tourniquet time in minutes (“time”) for each patient as a representation of operative time; decrease in tourniquet time represents learning. In addition, the following demographic variables are recorded: patient age, sex, side of surgery (right or left), patient height in inches, and patient weight in pounds.

Statistical Methods

Mean time, percentage of men, percentage of right knees, height, and weight were analyzed for consecutive blocks of 10 surgeries. To determine differences in the primary outcome measure among the 10 blocks, mean time was analyzed using analysis of variance (ANOVA) with Duncan’s multiple range test. Differences in age, height, and weight among blocks were assessed using ANOVA. Differences in sex and operative side among blocks were assessed using chi-square analysis. P<.05 were considered statistically significant. Statistical analyses were performed using SAS version 9.1.3 (SAS Institute; Cary, NC). In addition, time was analyzed using a logarithmic trend curve. Time was also analyzed using a best-fit linear equation (y = mx + b) for the entire cohort of 100 patients, where x represents patient number, y represents rotator cuff repair time (in minutes), and m, the slope, illustrates the rate of decrease in time as experience is gained (learning). Finally, time was also analyzed using a best-fit linear equation (y = mx + b) comparing the initial 10 patients with the subsequent 90 patients. Figures were created using Excel (Miscrosoft, Redmond, Wash).

Results

Mean patient age was 65 years (range: 41-83 years). Thirty-four percent of patients were men, and 53% of the operations were performed on the right knee. Mean patient height was 65 inches (range: 60-77 inches). Mean patient weight was 184 pounds (range: 104-304 lbs). Mean time was 79 minutes (range: 53-120 minutes).

Demographics and mean for consecutive blocks of 10 patients is illustrated in the Table. With regard to demographics, no statistically significant differences between consecutive blocks of 10 patients were observed for age (P=.73), sex (P=.21), operative side (P=.11), or weight (P=20). A significant difference in patient height was observed among the 10 blocks (P=.034). Patients in blocks 4 and 6 were approximately 2 inches taller than patients in the remaining blocks. However, when corrected for multiple statistical testing, the difference was not significant.

A significant difference in time was observed among the 10 blocks (P=.0025). The operative duration of block 1 was significantly greater than the operative duration of the remaining 9 blocks, and no statistically significant differences between the time of the remaining 9 blocks was observed. When corrected for multiple statistical testing for primary outcome measure, this difference is still significant.

Time by patient number is illustrated in Figure 1, in which best-fit linear trend lines are applied separately to the first block of 10 patients and the subsequent 90 patients. Time by patient number is again illustrated in Figure 2, in which a best-fit linear (as opposed to logarithmic) trend line is applied. The slope (m) of the line is -0.079, which represents a gradual rate of decrease in time (learning).

In Figure 3, best-fit linear (as opposed to logarithmic) trend lines are separately applied to the first block of 10 patients and the subsequent 90 patients. The slope (m) of the line fitting the first block of 10 patients is 1.76; the slope (m) of the line fitting the subsequent 90 patients is 0.02.

Discussion

The purpose of this study is to quantitate the number of surgeries required for a surgeon to become proficient in performing minimally invasive TKA. Operative time was significantly slower when comparing the first block of 10 patients with the remaining blocks of 10 patients (Table). Visual interpretation of the learning curve demonstrates rapid learning during the first 10 (or 11) patients compared with gradual learning during the remaining patients (Figure 1).

Quantitatively, learning occurred gradually throughout the study (m = -0.079; Figure 2). More specific quantitative analysis demonstrates that rate of change in operative time increased notably during the first 10 patients (m = 1.76; Figure 3) and almost not at all in the last 90 patients (m = 0.02; Figure 3). The investigators interpret this more specific quantitative analysis to demonstrate “struggling” with the learning curve during the first 10 patients, and that the learning curve was essentially completed during the subsequent 89 (Figure 1) or 90 (Figure 2) patients.

The investigators’ results support the hypothesis that with increased surgical experienced, learning may be demonstrated as a quantitative decrease in operative time.

A review of the minimally invasive TKA literature reveals that a learning curve has been reported previously^7-10 but not quantified. However, a broader review of the orthopedic literature reveals quantitation of the surgical learning curve. Surgeons performing a Bernese periacetabular osteotomy at an academic medical center reported that 9 of 10 major complications and all 4 failures occurred in the first 30 of 83 patients.¹¹ This study differs from the current study in both outcome measure (complications vs operative duration) and in learning curve (30 vs approximately 10 patients). Guttmann et al¹² reported the learning curve for arthroscopic rotator cuff repair and noted that rapid learning occurred during the first 10 patients. This study supports the investigators’ finding of a learning curve (as quantitated by rate of change of operative duration) of approximately 10 patients.

A limitation of this study is that clinical outcome is not necessarily proportional to operative time. Although it may represent learning, faster surgery does not imply better surgery. However, the literature supports minimally invasive TKA with regard to outcome.^1-4,6-9

Other limitations warrant consideration. Susceptibility bias is inevitable as different patients have different pathology and morphology. In addition, all cases were performed by a single surgeon, and several surgical assistants were used, which introduces performance bias. These biases are mitigated by evaluation of a large cohort of patients. A block size of 10 patients is arbitrary, but it is mathematically convenient and synchronous with visual interpretation of the results (Figure 1).

Although the number of repetitions required to achieve proficiency in performing minimally invasive TKA will vary, surgeons developing this technique may consider participating with more experienced surgeons or performing cadaveric surgical learning during their first 10 surgeries.

Conclusion

The learning curve for minimally invasive TKA approximates 10 patients, and operative time decreases as surgical experience is gained.

References

1. Laskin RS, Beksac B, Phongjunakorn A, Pittors K, Davis J, Shim JC. Minimally invasive total knee replacement through a mini-midvastus incision: an outcome study. Clin Orthop Relat Res. 2004; 428:74-81.
2. Haas SB, Cook S, Beksac B. Minimally invasive total knee replacement through a mini midvastus approach: a comparative study. Clin Orthop Relat Res. 2004; 428:68-73.
3. Haas SB, Manitta MA, Burdick P. Minimally invasive total knee arthroplasty: the mini midvastus approach. Clin Orthop Relat Res. 2006; 452:112-116.
4. Laskin RS. Minimally invasive total knee replacement using a mini-mid vastus incision: technique and results. Surg Technol Int. 2004; 13:231-238.
5. Tria AJ Jr. Minimally invasive total knee arthroplasty: the importance of instrumentation. Orthop Clin North Am. 2004; 35:227-234.
6. Laskin RS. Minimally invasive total knee arthroplasty: the results justify its use. Clin Orthop Relat Res. 2005; 440:54-59.
7. Bonutti PM, Mont MA, McMahon M, Ragland PS, Kester M. Minimally invasive total knee arthroplasty. J Bone Joint Surg Am. 2004;86(suppl 2):26-32.
8. Tanavalee A, Thiengwittavaporn S, Itiravivong P. Results of the 136 consecutive minimally invasive total knee arthroplasties. J Med Assoc Thai. 2005; 88(suppl 4):S74-S78.
9. Ohnsorge JA, Laskin RS. Special surgical technique of minimally invasive total knee replacement [in German]. Z Orthop Ihre Grenzgeb. 2006; 144:91-96.
10. Reid JB III, Guttmann D, Ayala M, Lubowitz JH. Minimally invasive surgery: total knee arthroplasty. Arthroscopy. 2004; 20:884-889.
11. Peters CL, Erickson JA, Jines JL. Early results of the Bernese periacetabular osteotomy: the learning curve at an academic medical center. J Bone Joint Surg Am. 2006; 88:1920-1926.
12. Guttmann D, Graham RD, MacLennan MJ, Lubowitz JH. Arthroscopic rotator cuff repair: the learning curve. Arthroscopy. 2005; 21:394-400.

Authors

Drs Lubowitz and Sahasrabudhe are from Taos Orthopedic Institute Research Foundation, Taos, NM, and Mr Appleby is from Smith & Nephew, Andover, Mass.

Dr Lubowitz is a consultant for Smith & Nepphew and has received grant funding from the sponsor within the past 12 months, Dr Sahasrabudhe has no financial relations with materials mentioned herein, and Mr Appleby is an employee of Smith & Nephew.