Reconstruction of the anterior cruciate ligament: dynamic strain evaluation of the graft

Abstract

The study is focused on the biomechanical aspects of the anterior cruciate ligament (ACL) reconstruction procedures with an emphasis on evaluating the dynamic strain of materials commonly used for this purpose. Separate and multiple, equally tensioned strands of hamstring grafts used for the reconstruction of the ACL were biomechanically tested and compared to original ACL and bone-patellar tendon-bone (BPTB) grafts, using tissue samples from cadavers. The study was focused on measuring such material properties as the strength, stiffness, maximum load, and elongation at maximum load of the original ACL, BPTB graft, and single tendon hamstring (gracilis and semitendinosus) grafts, continued by double strands and finally by four-strand graft (STG) evaluation. Fresh-frozen cadaveric knees were used, which had been clamped and tensioned equally. The measurement was performed by drop-weight testing, using a Laser Doppler Vibrometer as a basic sensor of the dynamic movements of the gripping clamps, with parallel correlation by a piezoelectric transducer. The grafts for experiments were obtained from 21-paired knees. The measurement was performed at room temperature (21 degrees C) after 24 h of thawing at 4 degrees C. All the specimens were measured for their response to the dynamic tensile load. The maximum strength values were obtained and calculated for the appropriate section area of the specimen. The tensioned strands of the original ACL showed a maximum average load of 1,246 +/- 243 N in the section area of about 30 mm(2) (max. stress 41.3 MPa); the strands of BPTB grafts showed values of 3,855 +/- 550 N in the section area of 80 mm(2) (max. stress 40.6 MPa); the gracilis tendons showed 925 +/- 127 N in the section area of 10 mm(2) (max. stress 95.1 MPa) and the semitendinosuss yielded a result of 2,050 +/- 159 N in the area of 20 mm(2) (max. stress 88.7 MPa). Of all the materials, the original ACL have the lowest strength and stiffness in respect of their biomechanical properties. BPTB grafts showed a slightly higher value of maximum stress, while both the gracilis and semitendinosus tendons showed double the value of maximum load per section area-tensile stress. Two- and four- combined hamstring strands clamped together and equally tensioned with a drop-weight had the combined tensile strength properties of the individual strands within the estimated range of measurement errors. No significant changes in maximum loads/stresses were observed under impact loading conditions. The results of this study demonstrate that equally tensioned four-strand hamstring-tendon grafts have higher initial tensile properties than those in other varieties of samples. From a biomechanical point of view, they seem to be a reasonable alternative procedure for ACL reconstruction.

Statistics in Brief: The Importance of Sample Size in the Planning and Interpretation of Medical Research

“…However, there is no statistical reason a significant result in a trial including 2000 patients should be given more belief Fig. 2 The graph shows the predicted confidence interval for experiments with an increasing number of specimens tested based on the study by Handl et al [11] of 21 fresh-frozen cadavers with a mean ultimate load failure of four-strand hamstring tendon constructs of 4546 N and standard deviation of 1500 N. than a trial including 20 patients, given the significance level chosen is the same in both trials. Small but wellconducted trials may yield a reliable estimation of treatment effect.…”

“…The precision of an estimation corresponds to the width of the confidence interval: the larger the tested sample size is, the better the precision. For instance, Handl et al [11], in a biomechanical study of 21 fresh-frozen cadavers, reported a mean ultimate load failure of four-strand hamstring tendon constructs of 4546 N under loading with a standard deviation of 1500 N. Based on these values, if we were to design an experiment to assess the ultimate load failure of a particular construct, the precision around the mean at the 95% confidence level would be expected to be 3725 N for five specimens, 2146 N for 10 specimens, 1238 N for 25 specimens, 853 N for 50 specimens, and 595 N for 100 specimens tested (Appendix 2); if we consider the estimated mean will be equal to 4546 N, the one obtained in the previous experiment, we could obtain the corresponding 95% confidence intervals (Fig. 2).…”

“…For example, Handl et al [11] in a biomechanical study of 21 fresh-frozen cadavers reported a mean ultimate load failure of 4-strand hamstring tendon constructs of 4546 N under dynamic loading with standard deviation of 1500 N. If we were to plan an experiment, the anticipated precision of the estimation at the 95% level would be 2 …”

ACL Strain and Tensile Forces for Weight Bearing and Non—Weight-Bearing Exercises After ACL Reconstruction: A Guide to Exercise Selection

“…Experimental biomechanical knee models [7][8][9][10][11][12][20][21][22][23][24][25][26][27][28] have several advantages. They allow a wide range of exercises with a wide range of external resistance and knee ranges of motion to be studied, they are inexpensive and noninvasive, and the estimated loads are better generalized to the active athletic population, because variables related to how the exercises were performed are often better controlled.…”

“…7,8,22 However, in the first few months post-ACL reconstruction, the ACL graft and the graft fixation sites are potentially significantly weaker than their eventual ultimate strength and may potentially be injured with considerably less force. Immediately after surgery, the graft fixation sites require time for incorporation with the surrounding bone.…”