Biomechanical and histological evaluation of the doubled semitendinosus tendon autograft after anterior cruciate ligament reconstruction in sheep

Kondo E, Yasuda K, Katsura T, Hayashi R, Kotani Y, & Tohyama H. The American Journal of Sports Medicine 2012. 40(2):315-324.

Anterior cruciate ligament (ACL) reconstruction often involves the use of a graft to help restore the knee to a point of normalcy.  However, the graft may not actually be “normal” either mechanically or histologically.  Animal reconstruction models allow us to closely evaluate the reconstructed graft but few have investigated the semitendinosus graft.  The purpose of this study was to evaluate the biomechanical and histological properties of semitendinosus ACL autografts in a sheep model.  The surgically reconstructed semitendinosus ACL autograft of 36 sheep were used for biomechanical (i.e., translation, tensile strength; 20 sheep) and histological (looking for cell structure and fiber alignment; 16 sheep) evaluation in comparison to contralateral control ACLs.  Five sheep were biomechanically evaluated at each of the 4 time points, 0, 12, 24, and 52 weeks post-surgery.  For histological evaluation, 3 sheep were evaluated at 4 time points: 0, 2, 24, and 52 weeks post-surgery, and 2 were evaluated at 6 and 12 weeks post-surgery.  Anterior-posterior translation was significantly greater in the ACL reconstruction knee than the contralateral control knees at all time points (0, 12, 24, & 52 weeks post-surgery), and translation was significantly less at 52 weeks than at 12 weeks post-surgery.  All ACL autografts failed tensile strength tests with 3 out of 5 being midsubstance tears at 12 weeks, and all 5 samples at 24 as well as 52 weeks post-surgery being midsubstance tears.  Maximum load at failure was significantly greater at 52 weeks than 12 weeks post-surgical.  In contrast, the contralateral control ACLs failed by avulsing from the bone. Histological observation was near normal by 24 weeks with abnormalities at earlier time points (0 and 12 weeks post-surgery); including the presence of necrotic (dead) tissue at 12 weeks.  Cell density was significantly lower at 2 weeks than all other time points.

Clinically, it appears that up to 1-year post-injury the semitendinosus autograft may not fully reach “normal” ACL biomechanical properties, despite histological similarities around this time point.  The authors note that these findings are similar to other autografts (e.g., bone-patellar tendon-bone).  Abnormalities that exist up to 1 year post-surgery have major implications regarding rehabilitation and return-to-play expectations.  There may be activity modifications or changes in rehabilitation programs that are necessary to promote the long-term health of the knee.  Based on the data, the authors suggest that vigorous activity should be limited in earlier periods when the graft is necrotized and weakened.  Even later in the process, the autografts are failing with midsubstance tears indicating that they are not as strong as the contralateral ACL.  As clinicians, we may need to think about acting more conservatively.  Research has demonstrated within humans (Beynnon et al. 2005) that there are biochemical abnormalities in the knee still present at 1-year post ACL reconstruction regardless of ACL rehabilitation program (conservative versus accelerated programs).  Within our current treatment approaches, it appears that the surgery (or the injury) itself may immediately alter the biomechanics and biochemistry of the knee and it is unclear when the graft returns to normal (if it ever does).  So the question remains, does the knee ever fully return to “normal” or do we need to establish a newly accepted “normal?”  Also, thinking long-term knee health, do we think it is healthy to return someone to activity with abnormalities or are we are returning athletes to activity prematurely after ACL reconstruction?

Written by: Nicole Cattano

Kondo E, Yasuda K, Katsura T, Hayashi R, Kotani Y, & Tohyama H (2012). Biomechanical and histological evaluations of the doubled semitendinosus tendon autograft after anterior cruciate ligament reconstruction in sheep. The American Journal of Sports Medicine, 40 (2), 315-24 PMID: 22088579