Pull-Out Strength and Stiffness of Meniscal Repair
February 1, 2000 4 minutes read
Pull-Out Strength and Stiffness of Meniscal Repair
Abstract & Commentary
Synopsis: The pull-out strength and stiffness for meniscal arrows were directly related to the number of barbs inserted across the repair and angle of insertion.
Source: Boenisch UW, et al. Pull-out strength and stiffness of meniscal repair using absorbable arrows or Ti-Cron vertical and horizontal loop sutures. Am J Sports Med 1999;27(5): 626-631.
Meniscal repair using bioabsorbable polylactic acid implants has became increasingly popular due to the ease of insertion with a fully arthroscopic technique (all-inside repair). Previous studies that examined pull-out strengths with the meniscal arrow implants did not specifically assess the influence of arrow length and angle of insertion. This study by Boenisch and colleagues aimed to determine how the number of barbs across the tear and the direction of insertion influence repair strength.
Seventy fresh frozen bovine menisci were divided into groups of 10 for pull-out studies. A template was used to create vertical, longitudinal tears. Repair techniques included horizontal loop suture with 2-0 nonabsorbable Ti-Cron sutures, vertical loop sutures, and meniscal arrows of 10-, 13-, and 16-mm lengths. Two other groups included an arrow insertion site closer to the tear to achieve more barbs across the tear, and an arrow direction of insertion angled 30° toward the tibial surface. Loads to failure were calculated at a constant displacement of 12.5 mm/s on an Instron machine.
As expected, the vertical suture construct was strongest (72 N), followed by the horizontal suture (68 N). The arrow pull-out strengths followed in order according to lengths for 16 mm (53 N), 13 mm (39 N), and 10 mm (19 N). Statistical significance was achieved between each group except between the two suture constructs.
Linear stiffness calculated from the load deformation curves was greatest for vertical sutures (12 N/mm), followed by 16-mm arrows (11 N/mm), 13-mm arrows (10 N/mm), horizontal sutures (8 N/mm), and 10-mm arrows (7 N/mm).
Mode of failure varied with arrow length. Longer arrows failed by pull-through of the T-head within the meniscus, while shorter arrows failed by barb pull-out from the periphery. Moving the insertion point to within 2 mm of the tear, compared to 5 mm, increased pull-out strengths for the 10-mm arrow from 10 N to 35 N. Insertion of arrows at a 30° angle significantly decreased the pull-out strength for 13-mm arrows from 39 N to 18 N. For all parameters there was a direct correlation between the number of barbs across the tear and pull-out strength. However, angled insertion was always weaker, despite an equal number of barbs across the tear.
Comment by David R. Diduch, MS, MD
The scientific strength of this paper reflects the long history of meniscal studies by one of the authors, Steve Arnoczky, DVM. Age-matched bovine menisci were used to reduce variability due to meniscal degeneration, and loading rates approximated in vivo forces. Tear length, direction of arrow insertion, and number of barbs across the repair were well controlled, and sample sizes were of appropriate size to detect statistical significance.
Boenisch et al provide the surgeon with helpful information for the use of the arrows. To improve the strength of the repair, the maximum number of barbs should engage the peripheral meniscal rim. If a shorter arrow is used, it should enter the meniscus closer to the tear. Angled insertion is to be avoided since it is much weaker, apparently due to the inability to capture the circumferential fibers of the meniscus.
Although the vertical suture technique was strongest, it was not much different than horizontal sutures or 16-mm arrows. Stiffness of vertical sutures, 16-mm and 13-mm arrows, was much better than that of horizontal sutures. Combining this information, Boenisch et al conclude that properly inserted arrows, with maximum number of barbs across the tear, should be sufficiently stable.
Of course, as an in vitro study, these results do not correlate with actual healing rates. The study also does not address the most common clinical scenario involving multiple arrows or sutures for a single repair. One logically can ask, how strong must the repair be? Only clinical studies can answer this question.
Because healing rates vary for meniscal repairs done in conjunction with an ACL reconstruction (about 85-90%) compared to an isolated repair (50-65%), choice of implant may be dictated by the clinical scenario. Vertical sutures, with greatest strength and stiffness, may be the best choice for isolated meniscal repairs, and offer the opportunity to use nonabsorbable material, which may be important should the repair only partially heal. Arrows, or other absorbable implants, seem quite appropriate for repairs done with an ACL reconstruction when the healing environment is optimal. Repairs done in an ACL-unstable knee are unlikely to heal regardless of the choice of implant.
These arrows and other new, absorbable implants have rapidly gained acceptance in the orthopaedic marketplace because of relatively easy insertion and minimal morbidity. Now it is time for clinical studies to help us determine their proper place.
Which construct for meniscal repair was strongest and stiffest?
a. 13-mm arrows
b. 16-mm arrows
c. Horizontal sutures
d. Vertical sutures
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