Assessment of Quadriceps Corticomotor and Spinal-Reflexive Excitability in Individuals with a History of Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis

Background Differences in the excitability of motor generating neural pathways are reported following anterior cruciate ligament reconstruction (ACLR) that is associated with quadriceps dysfunction and theorized to prevent the full recovery of muscle function. Objective The aims of this systematic review and meta-analysis were to compare quadriceps neural excitability between the involved ACLR limb, the uninvolved limb, and uninjured controls, and to determine at what time intervals these differences are present after surgery. Methods We conducted a search of PubMed, SPORTDiscus, Embase, and Web of Science, and extracted measures assessing difference of quadriceps spinal-reflexive, corticospinal, and intracortical excitability from studies that compared (1) involved limb to the uninvolved limb, (2) involved limb to a control limb, or (3) uninvolved limb to a control limb. We stratified time at 24 months, since this represents a period of heightened risk for reinjury. A modified Downs and Black checklist and Egger’s test were used to determine the methodological quality of individual studies and risk of bias between studies. Results Fourteen studies comprising 611 participants (371 individuals with a history of ACLR; median time from surgery: 31.5 months; range 0.5–221.1 months) were included in the review. Overall, the involved ( g = 0.60, 95% CI [0.24, 0.96]) and uninvolved ( g = 0.49, 95% CI [0.00, 0.98]) limbs exhibited greater motor threshold (MT) in comparison to uninjured controls. Motor-evoked potential (MEP) amplitudes were greater in the uninvolved limb in comparison to uninjured controls ( g = 0.31, 95% CI [0.03, 0.59]). Lesser intracortical inhibition was exhibited in the uninvolved limb compared to uninjured controls ( g = 0.54, 95% CI [0.14, 0.93]). When stratified by time from surgery, MEP amplitudes were greater in the uninvolved limb compared to uninjured controls ( g = 0.33, 95% CI [0.03, 0.63]) within the first 24 months after surgery. When evaluated more than 24 months after surgery, the involved limb exhibited greater Hoffmann reflex (H-reflex) compared to uninjured controls ( g = 0.38, 95% CI [0.00, 0.77]). MT were greater in the involved limb ( g = 0.93, 95% CI [− 0.01, 1.88]) and uninvolved limb ( g = 0.57, 95% CI [0.13, 1.02]) compared to uninjured controls. MEP amplitudes in the involved limb were lesser compared to uninjured controls when evaluated more than 24 months after ACLR ( g = -1.11, 95% CI [− 2.03, − 0.20]). Conc