What surgical technique for a total knee replacement causes the least amount of pain?

Many techniques for a total knee replacement have been tried since its beginnings in the 1970s.  

The standard approach is called a median parapatellar technique.  An incision is made with the scalpel on the inside of the knee cap to expose the knee joint.  The incision extends up the thigh into the quad muscle ("quad" stands for 4 and there are actually 4 muscles that make up the quad).  In this standard surgical approach to the knee, the Vastus Medialis is cut away from the Rectus Femoris (these are 2 of the 4 muscles, the other two are called the vastus intermedius and the vastus lateralis).  When closing the incision these muscles are stitched back together, but some people believe that avoiding an incision that separates these muscles may allow for less pain after surgery and faster recovery.

There are two "minimally invasive" surgical techniques that have been developed for a total knee replacement.  The mini mid-vastus approach mades a small incision into the Vastus Medialis but does not separate it from the Rectus femoris and primary quad insertion into the patella.  The subvastus avoids any cut into the quad muscle itself and is limited to the fascia.  The patella (knee cap) is not everted or even dislocated until the final insertion of implants.  The theory behind these "minimally invasive" approaches makes a lot of sense, but does the reality of clinical outcomes support the use of these techniques? Overall the definition of minimally invasive is a smaller skin incision (8-13 cm) and preservation of the insertion of the Vastus Medialis onto the patella. 

There does not appear to be a significant and reproducible benefit to any one approaches.   This suggests that most of the benefits of the surgical approach remain theoretical, and the factors that truly affect rehabilitation and overall function after surgery are less associated with the approach the visualizing the knee joint and probably more related to the cuts made into the bone and the implementation of the metal knee.  

All things considered, the advances in overall pain control using blocks and multi-modal pain medication are the most influential factor for improving function after Total Knee Replacement.  All of the approachs can produce equally good outcomes as long as the surgeon knows what he is doing. 

References. 

1.  Comparison of mini-midvastus and conventional total knee arthroplasty with clinical and radiographic evaluation: a prospective randomized clinical trial with 5- year followpup. JBJS 2016 Jun 15; 98(12): 1014-22. . see full article. no difference in clinical outcomes between mini midvastus and standard parapatellar approaches. 

2. Laskin RS et al. Minimally invasive total knee replacement through a mini-midvastus incision: an outcome study. CORR 2004 Nov; (428): 74-81. see full article.  this paper of only 38 patients suggests improved early function with 12 cm incision (vs. 18 cm in standard). 

3. Wülker N, Lambermont JP, Sacchetti L, Lazaró JG, Nardi J. A prospective randomized study of minimally invasive total knee arthroplasty compared with conventional surgery. J Bone Joint Surg Am. 2010 ;92(7):1584–90. see full article. no advantage.

4. Varela-Egocheaga JR, Suárez-Suárez MA, Fernández-Villán M, González-Sastre V, Varela-Gómez JR, Rodríguez-Merchán C. Minimally invasive subvastus approach: improving the results of total knee arthroplasty: a prospective, randomized trial. Clin Orthop Relat Res. 2010 ;468(5):1200–8. see full article. no advantage. 

5. Minimal incision total knee arthroplasty: early experience. Tria AJ Jr, Coon T. Clin Orthop Relat Res. 2003 Nov; (416):185-90. quad sparing technique. benefit in short term. see full article. 

6. Surgical approaches in mini-incision total knee arthroplasty. Scuderi GR, Tenholder M, Capeci C. Clin Orthop Relat Res. 2004 Nov; (428):61-7. see full article. benefit in short term. 

7. Lonner JH. Minimally invasive approaches to total knee arthroplasty: results. Am J Orthop. 2006; Jul: 35. see full article. review article.

8. Scuderi GR et al. Surgical approaches in mini-incision total knee arthroplasty. CORR 2004; 428: 61-67. see full article. review article. 

8. Anatomy of the extensor mechanism in reference to quadriceps-sparing TKA. Pagnano MW, Meneghini RM, Trousdale RT Clin Orthop Relat Res. 2006 Nov; 452():102-5. see full article.