Background: Quadriceps weakness is a hallmark of total knee arthroplasty and is driven by reduced voluntary muscle activation following the surgical procedure. The mechanisms underlying postoperative activation deficits are not well established, although nociception has been implicated via both spinal reflex and supraspinal pathways. The purpose of this study was to assess the role of nociception in postoperative recovery of strength and activation.
Methods: A total of 53 participants were assessed prior to total knee arthroplasty and at 6 weeks postoperatively. Quadriceps strength was measured by maximum voluntary isometric contraction, and activation was measured by the doublet interpolation technique. The pressure-pain threshold was used to measure local sensitization (at the knee joint) and systemic sensitization (at the forearm). Changes in outcomes (strength and activation) were regressed against pressure-pain threshold measurements. Mediation analyses were planned for significant associations to investigate whether deficits in voluntary activation were implicated on a causal pathway between pressure-pain threshold measures and postoperative strength loss.
Results: Knee pressure-pain threshold measures were significantly associated with reduced voluntary quadriceps activation (beta = -0.04; p = 0.009) and diminished quadriceps strength after total knee arthroplasty (beta = -0.07; p = 0.001). There was also a mediation effect of voluntary activation on the relationship between the knee pressure-pain threshold and quadriceps strength. After correcting for multiple comparisons, relationships between the forearm pressure-pain threshold and strength and activation did not reach significance.
Conclusions: The measures of local nociceptor sensitization were related to reduced strength and activation following total knee arthroplasty. This is consistent with a causal pathway linking increased firing of knee joint nociceptors to reduced activation and reduced strength. Future randomized studies should investigate whether peripherally directed pain therapies reduce pain while also promoting the recovery of quadriceps strength via an improved capacity for voluntary activation.
Level of evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.