Background & Aims

Knee osteoarthritis (OA) is a chronic pain disorder managed by total knee replacement (TKR) at the end stage. While TKR improves symptoms for many people, 20% of patients develop chronic postsurgical pain. Yet, mechanisms underlying the development of chronic postsurgical pain are unclear. Altered primary motor cortex (M1) function is linked to motor dysfunction and pain severity in chronic pain conditions, likely contributing to the development of chronic postsurgical pain in TKR. Using transcranial magnetic stimulation (TMS), research found reduced M1 excitability and an anterior shift in M1 representations for the quadriceps in chronic patellofemoral pain that could explain chronic pain and motor dysfunction in this condition. It is likely that similar M1 changes exist in knee OA and play a role in the development of chronic postsurgical pain in TKR. We aimed to examine M1 representations of the quadriceps muscle: 1) between people with and without knee OA; 2) before and after TKR.

Methods

M1 representations of the quadriceps muscle were examined cross-sectionally (20 people scheduled for TKR within 6 weeks and 20 age- and sex-matched controls) and longitudinally at 3-month intervals (TKR cohort; baseline (pre-TKR), and 3 and 6 months post-TKR) using rapid TMS mapping (126 TMS pulses). Peak-to-peak amplitude of electromyographic responses at the rectus femoris (RF), vastus lateralis (VL) and vastus medialis oblique (VMO) was measured to produce topographical representations (maps) of M1 projections to these muscles. Map volume (M1 excitability) and centre of gravity (CoG, location of M1 representation) were calculated. Average pain in the past week was measured by an 11-point numeric rating scale (NRS). Linear mixed-effects models were used to compare map parameters between knee OA and control groups, and changes in map parameters after TKR between those recovered (NRS?3) and unrecovered (NRS?4) at 6 months. Multiple comparisons were corrected and alpha was set at 0.05.

Results

People with knee OA had a smaller map volume in the M1 representations for the VMO (t71.2=2.25, adjusted p=0.03) compared with controls, but not for the RF or VL (adjusted p>0.07). The CoG for the RF (t49.4=2.51, adjusted p=0.02) and VL (t49.4=2.15, adjusted p=0.04) was located more posteriorly in people with knee OA when compared with controls. After TKR, map volume for the RF (t32.7=-2.77, adjusted p=0.02) and VL (t32.5=-3.50, adjusted p<0.01) increased at 3 months in the recovered participants. Map volume did not change over time for any muscles in the unrecovered participants (adjusted p>0.06). Further, map volume for the VL was greater (t40.1=2.38, adjusted p=0.02) in the recovered than unrecovered participants at 3 months after TKR. Compared with baseline, an anterior shift in the CoG for the RF was observed at 3 months in the recovered participants (t33.6=-2.48, adjusted p=0.047) and similar change was observed at 6 months in those unrecovered (t32.3=-2.67, adjusted p=0.03).

Conclusions

The cross-sectional data provide the first evidence that M1 function is altered in people with knee OA compared with controls. Notably, M1 changes differed between 3 quadriceps muscles, including lower M1 excitability (smaller map volume) for the VMO and more posteriorly located representations for the RF and VL in knee OA than controls. The longitudinal data provide the preliminary evidence that M1 changes after TKR may distinguish 6-month outcome. M1 representations of the quadriceps changed differently between people who did and did not recover at 6 months post-TKR. The recovered individuals had an increase in M1 excitability (greater map volume for the RF and VL) from baseline to 3 months post-TKR and had greater M1 excitability at 3 months than those unrecovered. Anteriorly shifted representation of the RF after TKR occurred earlier in those recovered than those unrecovered. These data provide novel insight into the pathophysiology of knee OA and chronic postsurgical pain in TKR.

References

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Presenting Author

Wei-Ju Chang

Poster Authors

Wei-Ju Chang

PhD

The University of Newcastle; Neuroscience Research Australia

Lead Author

Topics

  • Specific Pain Conditions/Pain in Specific Populations: Post-surgical/Post-traumatic Chronic Pain