Isometric testing finds weakness after hamstring graft ACL reconstruction

Despite the popularity of hamstring autograft in anterior cruciate ligament reconstruction (ACLR), questions exist about potential negative effects of autologous hamstring harvest (Fig. 1). Perhaps the most notable concern—the potential loss of knee flexion strength—is supported by the results of a study Kenneth R. Morse, MD, presented at the 2009 American Orthopaedic Society for Sports Medicine annual meeting.

In the study, isometric testing revealed a decrease in hamstring muscle strength in the involved extremity of patients who had undergone ACLR with hamstring autograft. Isokinetic testing did not find any significant differences.

“Based on these results,” said Dr. Morse, “we believe that isometric testing may isolate hamstring strength better than isokinetic testing. We also believe that postsurgical rehabilitation should emphasize hamstring strength more heavily than it currently does.”

Testing hamstring strength
Study participants included 14 patients (8 females and 6 males) between the ages of 18 and 30 years (median age, 22.5 years) who had undergone ACLR with autologous hamstring graft 1 to 2 years before the study.

Each patient completed a demographic questionnaire, the SF-36 General Health Status Questionnaire, and three outcomes surveys to rate their knee function. Researchers examined the patients to evaluate their passive range of motion, maximum standing knee flexion angle, and manual and instrumented ligamentous stability.

The vertical jump test and the one-legged hop test were used to assess functional strength. After patients warmed up for 5 minutes on a stationary bicycle and stretched, their hamstring and quadriceps torque in the involved and uninvolved extremity were measured isokinetically and isometrically, using a dynamometer.

“Hamstring and quadriceps torque was tested at 60 degrees per second and 180 degrees per second,” said Dr. Morse. “Isometric knee flexor strength was tested at 30 degrees and 90 degrees of flexion, in both the internally rotated and neutral positions (Fig. 2).”

He noted that isokinetic testing was performed in the neutral position because it is difficult to ensure an internally rotated position throughout the range of motion.

“To avoid altered results secondary to fatigue,” he added, “isometric and isokinetic testing were performed on separate days, one week apart, and the order was randomized.”

Different results for testing
Isokinetic testing did not reveal any statistically significant differences between the involved and uninvolved extremity. Isometric testing, however, found a substantial decrease in knee flexion strength.

“At an average follow-up of 1.5 years, testing revealed that the involved extremity had significantly less torque in all positions,” said Dr. Morse. “This difference was most pronounced at 90 degrees of flexion, where the isometric strength averaged 64.20 percent of the uninvolved extremity in the neutral position, and 65.22 percent of the uninvolved extremity in the internally rotated position.

“In all four isometric tests,” continued Dr. Morse, “more than 70 percent of patients had more than a 10 percent deficit in the involved extremity compared to the uninvolved limb (Table 1).”

No significant differences between the internally rotated and neutral positions were found at any flexion angle. Nor did researchers find a significant difference between the involved and uninvolved extremities during the hop and vertical jump tests.

Based on the study’s results, stated Dr. Morse, isometric strength testing may be a more sensitive means of measuring hamstring weakness after autograft harvest.

“In the future,” he said, “we will continue to test patients who had hamstring autograft, and compare their results to patients with patella tendon autograft as well as to those with allograft. We also think it’s important to evaluate the effects of a rehabilitation program that has a strong focus on improving hamstring strength.”

Dr. Morse noted that strength testing was not available before ACLR was performed, so it was not possible to evaluate whether isometric differences existed prior to surgery.

“In addition,” he said, “the follow-up of 1.5 years may be too short to see any difference that may exist with isokinetic testing. Finally, more patients may be needed to detect significant differences in the hop and vertical jump tests, although this was not the primary purpose of this study.”

Dr. Morse was the lead author of “Isokinetic and Isometric Assessment of Knee Flexor Strength following Autogenous Hamstring Graft Anterior Cruciate Ligament Reconstruction.” He reports no conflicts. Dr. Morse’s co-authors included Bryson P. Lesniak, MD; James J. Irrgang, PhD, PT, ATC; and Christopher D. Harner, MD.

Disclosure information: Dr. Lesniak—none; Dr. Irrgang—Biomet; DePuy, A Johnson & Johnson Company; Smith & Nephew; Stryker; and Synthes; and Dr. Harner—Smith & Nephew.

Jennie McKee is a staff writer for AAOS Now. She can be reached at mckee@aaos.org