Strong evidence supports that CPM after knee arthroplasty (KA) does not improve outcomes.

Rationale
Two high quality studies (Beaupre 2001, Denis 2006) and five moderate quality studies (Can 2003, Chen 2013, Herbold 2014, MacDonald 2000, Montgomery 1996) compared the utilization of continuous passive motion during hospital stay to no utilization of continuous passive motion. The combined results provide strong evidence that the surgical outcomes for those who used continuous passive motion are not better than for those who did not use continuous passive motion.   

Five of the seven studies measured outcomes of physical function and quality of life. Beaupre, Denis, Herbold, and MacDonald found no significant differences in a gamut of outcomes (WOMAC, SF-36, Timed “up + go” [TUG], functional independence measure [FIM], and Knee Society Score). Chen reported better quality of life in the group that did not use continuous passive motion. Knee range of motion was investigated by Beaupre, Denis, and Chen. Meta-analysis showed no differences in knee range of motion. Complications were evaluated by Beaupre and Denis and were not statistically different between groups. Beaupra, Can, Chen, MacDonald, and Montgomery demonstrated that pain and stiffness were not decreased by CPM, whereas Denis reported significantly less pain in the continuous passive motion group (12 points difference in VAS ranging from 0-100). Meta-analysis from Denis, Herbold, and Montgomery showed no differences in length of hospital stay. 

One high quality study (Lenssen 2008) demonstrated no statistically significant benefits in functional outcome scores or range of motion with the use of continuous passive motion in conjunction with physical therapy compared to physical therapy alone. The continuous passive motion was used for 17 consecutive days after surgery (about 2 weeks after discharge).
  1. Beaupre,L.A., Davies,D.M., Jones,C.A., Cinats,J.G. Exercise combined with continuous passive motion or slider board therapy compared with exercise only: a randomized controlled trial of patients following total knee arthroplasty. Phys.Ther. 2001; 0: 1029-1037
  2. Can,F., Alpaslan,M. Continuous passive motion on pain management in patients with total knee arthroplasty. Pain Clinic 2003; 4: 479-485
  3. Chen,L.H., Chen,C.H., Lin,S.Y., Chien,S.H., Su,J.Y., Huang,C.Y., Wang,H.Y., Chou,C.L., Tsai,T.Y., Cheng,Y.M., Huang,H.T. Aggressive continuous passive motion exercise does not improve knee range of motion after total knee arthroplasty. J Clin Nurs. 2013/2; 3: 389-394
  4. Denis,M., Moffet,H., Caron,F., Ouellet,D., Paquet,J., Nolet,L. Effectiveness of continuous passive motion and conventional physical therapy after total knee arthroplasty: a randomized clinical trial. Phys Ther 2006/2; 2: 174-185
  5. Herbold,J.A., Bonistall,K., Blackburn,M., Agolli,J., Gaston,S., Gross,C., Kuta,A., Babyar,S. Randomized controlled trial of the effectiveness of continuous passive motion after total knee replacement. Arch Phys Med Rehabil 2014/7; 7: 1240-1245
  6. Lenssen,T.A., van Steyn,M.J., Crijns,Y.H., Waltje,E.M., Roox,G.M., Geesink,R.J., van den Brandt,P.A., de Bie,R.A. Effectiveness of prolonged use of continuous passive motion (CPM), as an adjunct to physiotherapy, after total knee arthroplasty. BMC Musculoskelet.Disord. 2008; 0: 60-
  7. MacDonald,S.J., Bourne,R.B., Rorabeck,C.H., McCalden,R.W., Kramer,J., Vaz,M. Prospective randomized clinical trial of continuous passive motion after total knee arthroplasty. Clin Orthop Relat Res. 2000/11; 380: 30-35
  8. Montgomery,F., Eliasson,M. Continuous passive motion compared to active physical therapy after knee arthroplasty: similar hospitalization times in a randomized study of 68 patients. Acta Orthop Scand. 1996/2; 1: 7-9