Interference fixation versus postfixation of bone-patellar tendon-bone grafts for anterior cruciate ligament reconstruction. A biomechanical comparative study in porcine knees.

This study compares biomechanical characteristics of interference screw fixation to "postfixation" (sutures tied over cancellous screw and washer) of bone-patellar tendon-bone grafts in porcine anatomic specimen knees. Maximum pull-out strength (ultimate failure load), displacement of the bone graft at 110 N of force, and mode of failure were compared in a progressive load to failure test. Interference screw fixation demonstrated statistically significantly (p = 0.04) higher mean ultimate failure loads (535 N) compared to postfixation (309.1 N). Interference fixation also demonstrated statistically significantly (p = 0.0003) less displacement of the bone graft at 110 N of force (0.32 mm vs. 2.21 mm). All failures occurred at the fixation site. The authors conclude that interference fixation is stronger than postfixation and, in the immediate postoperative period, should better protect the graft from loosening after anterior cruciate ligament reconstruction, improving stability and allowing more aggressive rehabilitation.

A new canine cruciate ligament formed through distraction histogenesis. Report of a pilot study.

A modified Ilizarov external fixator was used to transfix the stifle joint in 13 dogs. A 1-cm cortical cancellous bone plug was outlined in the intracondylar notch of the femur after excising the anterior (cranial) cruciate ligament (ACL). Hardware attached to the plug allowed controlled distraction of the plug into a cut tibial channel trailing the regenerate ligament. No attempt was made to exactly recreate the normal anatomic course of the ACL. Some bone plugs became dislodged from the femur before distraction began, while others consolidated prematurely. These specimens produced scar only and served as a control group against which the regenerate ligament and normal ACLs were contrasted biomechanically, biochemically, and histologically. The dogs were killed at intervals from four to 21 weeks postoperatively. Load to failure at 21 weeks reached 71% of the mean normal. Histologic examination showed an improved fiber organization of the regenerate compared with the scar group. The regenerate and scar groups showed increased percent water compared with normal, while there was no significant difference in percent collagen and glycosaminoglycan content.

Effect of spinal construct stiffness on early fusion mass incorporation. Experimental study.

The relationship between initial spinal construct stiffness and the stiffness of the resulting fusion mass was studied by performing standardized 10-segment posterior spinal fusions in goats. Animals were divided into 5 groups based on type of spinal construct, using rods of different diameters (3.2 mm, 4.8 mm, 6.4 mm) with or without rigid crosslinking to produce constructs of different stiffnesses. Stiffness data on 28 animals were obtained by removing the spines en bloc, at 6 or 12 weeks postoperatively, and performing load-deformation testing in axial and torsional loading to determine the stiffness of the fusion masses (rods removed). The initial construct stiffnesses were also compared by ex vivo testing on spine specimens to correlate initial construct stiffness with eventual fusion mass stiffness. In axial testing, results showed stiffer fusion masses from larger diameter rod constructs compared with smaller rod constructs. This was similar to results of control testing on spine specimens ex vivo. Rigid crosslinking did not produce stiffer fusions in axial testing, due to a technical limitation of the button-wire implants used to segmentally fix the rods at each vertebra. In torsional testing, stiffer fusion masses resulted from using larger rods, and rigid Crosslinking also produced the stiffest fusion masses, which was consistent with ex vivo testing. In general, larger diameter (stiffer) rods produced stiffer fusion masses, and no evidence of stress shielding was found.

The strength of a posterior element claw at one versus two spinal levels.

In the Cotrel-Dubousset and Texas Scottish Rite Hospital spinal implants systems, the upper hook on the convex rod is usually clawed to prevent posterior pull out. A transverse process hook and an upwardly directed pedicle hook are usually inserted on the same lamina level. However, clawing two levels instead of one is a simpler surgical procedure. We biomechanically compared the posterior pull-out strength of a one-level versus two-level construct. The average load of failure of the double-level construct was significantly greater (p less than 0.05) as compared with the single level. Thus, the two-level construct is not only easier to insert, it is also stronger.