The basic science of continuous passive motion in promoting knee health: a systematic review of studies in a rabbit model

Knapik DM, et al. · Arthroscopy

Against immobilisation, CPM prevented joint stiffness - flexion contracture in 0-13% of CPM knees against 40-100% of immobilised knees - and produced better cartilage histology, with hyaline cartilage predominant in 70-100% of CPM defects against 8-79% of immobilised and 10-73% of freely moving ones. Against ordinary cage activity the functional advantage did not hold: no difference in contracture from 1 to 10 weeks, and none in any group at 52 weeks. The review found no nutritional benefit - CPM did not raise uptake of low-molecular-weight nutrients - and attributes the effect instead to sinusoidal intra-articular pressure changes that clear blood and solutes from the joint, and to suppression of interleukin 1, interleukin 6 and tumour necrosis factor alpha. This is the basic-science rationale, from Salter's work onward, behind CPM after cartilage repair; human benefit is a separate question.

Population

19 rabbit studies of continuous passive motion after chondral or osteochondral injury, drawn from 481 citations

Intervention

Continuous passive motion, 1 day to 4 weeks of therapy, for as much as 24 hours a day, over 40 to 130 degrees of motion

Comparator

Immobilisation, intermittent active motion (normal cage activity), or both

Limitations

A rabbit model, heterogeneous in how injuries were made and where they sat. Only one of the 19 studies followed beyond 12 weeks, and six of the nine osteochondral-defect studies used periosteal autograft, which the authors note is no longer in international use. Nothing here translates the basic science to human patients. One author declares support from Sanofi and Smith and Nephew.