The aim of this study was to describe the postoperative "baseline" magnetic resonance imaging (MRI) appearance of the ipsilateral thigh musculature after total knee arthroplasty (TKA). The secondary aim was to describe baseline muscle enzyme levels under the same clinical scenario. Neither of these measures have been reported previously.

• In 2016, 3 sentinel cases of presumed local anesthetic-induced myotoxicity were reported after TKA with associated continuous adductor canal block (CACB) analgesia. The diagnosis was made based on its consistency with animal and human descriptions of local anesthetic myotoxicity, including: a) exposure to local anesthetic, b) delayed symptom onset after a period of normal recovery, c) rapid development of profound muscle flaccidity, and d) complete to partial recovery of motor function after weeks to months. MRIs obtained in these patients showed diffuse intra- and inter-muscular edema (inflammation) of the ipsilateral thigh, which was believed to support the presumptive diagnosis. However, because baseline MRI appearance of upper leg muscles has never been described for this clinical scenario, it is possible that the MRI scans represented "normal, baseline" appearance.
• This case series intended to prospectively describe postoperative MRI appearance in a cohort of asymptomatic volunteer patients that underwent uncomplicated TKA/CACB. In addition, because obtaining biomarkers of muscle injury might also be a reasonable diagnostic step, we sought to measure preoperative and postoperative creatine phosphokinase (CPK) and aldolase levels.
• MRI scans were read by 5 board-certified musculoskeletal radiologists masked to the study's purpose. Grading was done using a standard grid that facilitated systematic evaluation of various regions within the upper leg. At least 3 of 5 radiologists were required to declare edema as present within a given region of the leg.
• Only those volunteer patients that presented a normal postoperative course, i.e., had no unexpected leg muscle weakness, were entered into the study. As such, the MRI and muscle enzyme analysis results had no impact on the volunteer patients' clinical outcome or management. This was a prospective, observational/descriptive case series. There was no control group and we did not intend to investigate issues of cause-and-effect.

Inclusion Criteria:

• Scheduled for unilateral primary TKA with CACB
• TKA and early recovery was uncomplicated (no evidence of unexpected leg weakness)

Exclusion Criteria:

• Contraindication to spinal anesthesia or adductor canal-based analgesia
• History of muscle wasting or related disease
• History of autoimmune disorders that may affect muscles
• History of neurologic condition affecting the lower extremities
• Contraindications to MRI

• Volunteer patients of various age and sex
• Number of volunteers and diagnostic studies determined by available grant monies
• Volunteers were drawn from a population presenting for unilateral primary TKA performed by 1 of 2 orthopedic surgeons in a single institution

De-identified raw data in Excel format may be obtained from Joseph M. Neal, MD upon request.

March 2021 through March 2026

Upon request

10 days. Participant was evaluated at the time of the incident and twice during the 10 days thereafter.

This case series aimed to describe MRI appearance and muscle enzyme levels presented under the same clinical conditions as the 3 sentinel cases of presumed local anesthetic myotoxicity after TKA/CACB. This descriptive design was not intended to sort cause-and-effect (that is, the relative role of surgery, pneumatic tourniquet, local anesthetic, etc.) in causing muscular edema or raising enzyme levels.