Abstract
Background
Hamstring autografts with a diameter of less than 8 mm for ACL reconstruction have an increased risk of failure, but there is no consensus regarding the best method to predict autograft size in ACL reconstruction.
Questions/purposes
(1) What is the relationship between hamstring cross-section on preoperative MRI and intraoperative autograft size? (2) What is the minimum hamstring tendon cross-sectional area on MRI needed to produce an autograft of at least 8 mm at its thickest point?
Methods
This was a retrospective cohort study of 68 patients. We collectively reviewed patients who underwent ACL reconstruction by three separate fellowship-trained surgeons at the Carilion Clinic between April 2010 and July 2013. We searched the patient records database of each surgeon using the keyword “ACL”. A total of 293 ACL reconstructions were performed during that time period. Of those, 23% (68 patients) had their preoperative MRI (1.5 T or 3 T magnet) performed at the Carilion Clinic with MRI confirmation of acute total ACL rupture. Exclusion criteria included previous ACL reconstructions, multiligamentous injuries, and history of acute hamstring injuries.
After applying the exclusion criteria, there were 29 patients in the 1.5 T magnet group and 39 in the 3 T group. Median age (range) was 29 years (12 to 50) for the 1.5 T group and 19 years (9 to 43) for the 3 T group. The patients were 41% female in the 1.5 T group and 23% female in the 3 T group. Use of 1.5 T or 3 T magnets was based on clinical availability and scheduling. The graft’s preoperative cross-sectional area was compared with the intraoperative graft’s diameter. The MRI measurements were performed by a single musculoskeletal radiologist at the widest point of the medial femoral condyle and at the joint line. Intraoperative measurements were performed by recording the smallest hole the graft could fit through at its widest point. Pearson’s correlation coefficients were calculated to determine the relationship between graft size and tendon cross-sectional area. A simple logistic regression analysis was used to calculate the cutoff cross-sectional areas needed for a graft measuring at least 8 mm at its thickest point. Intrarater reliability was evaluated based on re-measurement of 19 tendons, which produced an overall intraclass correlation coefficient (ICC) of 0.96 95% (CI 0.93 to 0.98). A p value < 0.05 was considered significant.
Results
In general, the correlation between MRI-measured hamstring thickness and hamstring graft thickness as measured in the operating room were good but not excellent. The three measurements that demonstrated the strongest correlation with graft size in the 1.5 T group were the semitendinosus at the medial femoral condyle (r = 0.69; p < 0.001), the semitendinosus and gracilis at the medial femoral condyle (r = 0.70; p < 0.001), and the mean semitendinosus and gracilis (r = 0.64; p < 0.001). These three measurements had correlation values of 0.53, 0.56, and 0.56, respectively, in the 3 T MRI group (all p values < 0.001). To create an 8-mm hamstring autograft, the mean semitendinosus plus gracilis cutoff values areas were 18.8 mm2 and 17.5 mm2 for the 1.5 T and 3.0 T MRI groups, respectively.
Conclusions
Imaging performed according to routine knee injury protocol can be used to preoperatively predict the size of hamstring autografts for ACL reconstructions. In clinical practice, this can assist orthopaedic surgeons in graft selection and surgical planning.
Level of Evidence
Level II, diagnostic study.
Introduction
ACL injuries are common in athletes and trauma patients. Hamstring tendons are a typical autograft choice in ACL reconstruction because they have been shown to result in similar stability and patient-reported outcomes to bone-patella-bone autografts without the associated donor site morbidity [17, 20], but one recognized potential limitation in the use of hamstring autografts is the variability in hamstring tendon size that exists in the population. Recent studies have found that hamstring autografts less than 8 mm have an increased risk of rerupture compared with thicker ones, especially in patients younger than 20 years old [6, 23, 24].
As the harvest of an inadequately sized graft may necessitate a change in the planned ACL reconstruction procedure (from a hamstring graft to a bone-patellar tendon-bone graft or to an allograft) so as not to use an undersized graft (with its attendant risk of graft failure), multiple studies have evaluated methods to predict hamstring autograft size preoperatively [2, 3, 31]. Unfortunately, studies looking at the relationship between intraoperative graft size and anthropometric parameters have been generally inconclusive [1, 4, 5, 11, 15, 19, 22, 26, 27]. Predictions using ultrasound have had similarly inconsistent results [8, 9, 25]. In contrast, recent studies evaluating the relationship between hamstring tendon size on MRI and intraoperative hamstring autograft size have produced promising results [8, 12, 13, 29]. Nevertheless, the methods used to determine tendon size in those studies were complex and not easily transferrable to routine clinical practice. We were interested to see whether an easier-to-use option involving routine preoperative MRIs could deliver in sufficient correlations between hamstring size the MRI and tendon size at the time of surgery.
The purpose of this study is to assess whether routine preoperative MRI is able to predict hamstring ACL graft size, and whether this could help to identify grafts of insufficient size.
We therefore asked (1) What is the relationship between hamstring cross-section on preoperative MRI and intraoperative autograft size? (2) What is the minimum hamstring tendon cross-sectional area on MRI needed to produce an autograft of at least 8 mm at its thickest point?
Patients and Methods
Using the keyword “ACL”, we retrospectively reviewed the records of patients who underwent ACL reconstruction surgery by one of three sports medicine-trained orthopaedic surgeons (BMJ, TKM, CKJ) at Carilion Clinic between April 2010 and July 2013. A total of 293 ACL reconstructions were performed during that time period. Patients were considered for inclusion if their preoperative MRI was performed at our institution using a routine acute knee injury MRI protocol (1.5 T or 3 T), described below, and they subsequently underwent ACL reconstruction using a hamstring autograft with a documented intraoperative diameter. Exclusion criteria included previous ACL reconstructions, multiligamentous injuries, and history of acute hamstring injuries. After applying the exclusion criteria, there were 29 patients in the 1.5 T group and 39 in the 3 T group. Median age (range) was 29 years (12 to 50) for the 1.5 T group and 19 years (9 to 43) for the 3 T group. The patients were 41% female in the 1.5 T group and 23% female in the 3 T group (Fig. 1). Use of 1.5 T or 3 T magnets was based on clinical availability and scheduling. Institutional review board approval was obtained for this study to allow access to patient data in the electronic health record.
Fig. 1.
This STROBE flowchart describes the distribution of patients during the study period.
Routine Acute Knee Injury MRI Protocol
The institution’s standard acute knee injury MRI protocol specifies the field of view, matrix size, and slice thickness. However, these parameters may have been modified to accommodate the patient’s body habitus and condition. The typical parameters for the turbo spin echo T2 fat-suppressed axial images used in this study were as follows: TR, 3600; TE, 50; field of view, 100 mm; matrix, 384 x 288; and slice thickness, 3 mm. Images were acquired with a dedicated multichannel knee coil designed for that specific MRI unit except when the patient’s size or condition would not allow placement of the knee coil.
Preoperative MRI Measurements
Preoperative imaging of all patients was performed using either a 3 T or 1.5 T Siemens (Munich, Germany) MRI machine at our institution, based on availability. Only images ordered by the surgeon for assessment of probable ACL injury or for preoperative assessment were used.
For each measurement, the gracilis and semitendinosus tendons were evaluated separately. We made measurements using the freehand region of interest tool in AGFA IMPAX (Mortsel, Belgium) workstation by manually tracing the tendon. Measurements were performed using axial images at the widest point of the medial femoral condyle and at the joint line under maximum magnification (Fig. 2A-D). The three measurements that demonstrated the strongest correlation with the intraoperative graft size were used to calculate cutoff values for preoperative measurements. All measurements were performed by a single fellowship-trained musculoskeletal radiologist (KKW) at our institution. The reporting radiologist did not have access to the intraoperative determination of graft size.
Fig. 2 A-D.
(A) Fat-saturated, proton density-weighted, axial MR images show the knee at the widest point of the medial femoral condyle. The semitendinosus and gracilis are indicated. (B) The semitendinosus and gracilis are shown at 2 x magnification. (C) The semitendinosus, measured at the medial femoral condyle at full magnification, is shown in this figure. (D) The gracilis, measured at the medial femoral condyle at full magnification, is shown.
Intraoperative Measurements
Intraoperative hamstring graft measurements were made at the time of surgery. The hamstring tendons were stripped from their insertion site using a fluted graft-harvesting device. The diameters of the gracilis and semitendinosus tendons were measured together (four-stranded) using a standardized graft-sizing block with holes of 0.5-mm increments. The diameter of the graft was determined by the smallest hole through which the graft could pass at its widest point.
Statistical Analysis
We performed statistical analysis using SPSS (IBM Corp, Chicago, IL, USA) and R Studio (R Studio Inc, Boston, MA, USA). Pearson’s correlation coefficients were calculated to determine the relationship between the semitendinosus, gracilis, and semitendinosus and gracilis at the widest point of the medial femoral condyle and joint line and the mean of both measurements. We used a simple logistic regression analysis to calculate the cutoff values needed to provide a graft size of 8 mm. The sensitivity, specificity, positive predictive value, and negative predictive value were calculated based on the minimum MRI cross-sectional area required to create an 8-mm graft. To evaluate intrarater reliability, the musculoskeletal radiologist remeasured 19 tendons, from which an overall intraclass correlation coefficient of 0.96 was produced (95% CI 0.93 to 0.98) (see Table, Supplemental Digital Content 1, http://links.lww.com/CORR/A227). A p value < 0.05 was considered significant.
Results
In general, the correlation between MRI-measured hamstring thickness and hamstring graft thickness as measured in the operating room were good but not excellent in the 1.5 T and 3 T groups. The three measurements that demonstrated the strongest correlation with the intraoperative graft size in the 1.5 T MRI group were the semitendinosus at the medial femoral condyle (r = 0.69; p < 0.001), the semitendinosus and gracilis at the medial femoral condyle (r = 0.70; p < 0.001), and the mean semitendinosus and gracilis (r = 0.64; p < 0.001) (Table 1). The three measurements that demonstrated the strongest correlation with the intraoperative graft size in the 3 T group were the semitendinosus at the medial femoral condyle (r = 0.53; p < 0.001), the semitendinosus and gracilis at the medial femoral condyle (r = 0.56; p < 0.001), and the mean semitendinosus and gracilis (r = 0.56; p < 0.001) (Table 2). Intraoperative graft sizes ranged from 6 mm to 10.5 mm with a mean intraoperative graft size of 8 mm.
Table 1.
Correlation coefficients, 95% CIs, and p values for each cross-sectional area as seen on 1.5 T MRI and intraoperative graft diameter
Table 2.
Correlation coefficients, 95% CIs, and p values for each cross-sectional area as seen on 3.0 T MRI and intraoperative graft diameter
In the 1.5 T MRI group, the minimum cross-sectional area needed to produce an autograft with an 8-mm diameter was 12.3 mm2 for the semitendinosus at the medial femoral condyle, 19.9 mm2 for the semitendinosus and gracilis at the medial femoral condyle, and 18.8 mm2 for the mean semitendinosus and gracilis. In the 3 T MRI group, the minimum cross-sectional area needed to produce an autograft with an 8-mm diameter was 11.4 mm2 for the semitendinosus at the medial femoral condyle, 18.3 mm2 for the semitendinosus and gracilis at the medial femoral condyle, and 17.51 mm2 for the mean semitendinosus and gracilis (p < 0.05) (Fig. 3). The sensitivity, positive predictive value, and negative predictive value calculated based on an 8-mm cutoff had the strongest values when the semitendinosus and gracilis at the femoral condyle were measured using 1.5 T MRI (92%, 80%, and 93%, respectively) (Table 3; see Table, Supplemental Digital Content 2, http://links.lww.com/CORR/A228). Specificity was best for the semitendinosus at the femoral condyle using 3 T MRI (88%) (Table 4; see Table, Supplemental Digital Content 3, http://links.lww.com/CORR/A229).
Fig. 3.
This figure shows the minimum 1.5 T (navy blue) and 3 T (light blue) MRI cross-sectional areas required to provide a hamstring autograft diameter of at least 8 mm. Using a simple linear regression analysis, we determined cutoff values for the three measurements that had the strongest correlation with the intraoperative graft size; ST = semitendinosus; GR = gracilis; FC = femoral condyle; AVG = (mean).
Table 3.
Sensitivity, specificity, positive predictive value, and negative predictive value to determine an 8-mm graft using 1.5 T MRI
Table 4.
Sensitivity, specificity, positive predictive value, and negative predictive value to determine an 8-mm graft using 3 T MRI
Discussion
Background and Rationale
The proportion of revision after ACL reconstruction is reported to range from 3.1% to 4.1% [7, 21]. According to some reports, as many as 8.4% of patients will have graft failure [14, 16] and undergo revision surgery [7, 10, 18, 21, 32]. Most graft failures are considered to be due to technical error [10, 18], including inadequate graft size. The current evidence supports the use of 8 mm as a cutoff [6, 23, 24]. We asked whether routine preoperative MRI could reliably predict graft size and avoid the use of insufficiently sized grafts. We found that 1.5 T and 3 T MRI were both able to be used for prediction, as they provided good correlations to intraoperative graft size. However, stronger correlation values were found for the semitendinosus and at the level of the medial femoral condyle.
Limitations
This study was limited by the fact that all measurements were made by a single musculoskeletal radiologist (KKW). Therefore, interobserver variability was not evaluated. Interobserver variability will be evaluated as a prospective arm of this measurement protocol. Nevertheless, the radiologist was shown to have excellent intraobserver reliability throughout the study, indicating consistency in his measurements. As with any retrospective study, there is concern of possible selection bias. However, each patient’s record was reviewed at the time of data collection to ensure that only patients who met our criteria were enrolled. It should be noted that this study was designed only to assess correlation and potential use for prediction of an 8-mm size cutoff for a four-strand/two-limb graft construct. For smaller patients, grafts less than 8 mm may still be appropriate and could be considered, just as this may be insufficient for much larger patients [30]. Finally, we did not attempt to correlate our findings on graft size with knee function, return to sport, or objective measures such as ROM or stability, as these outcomes were beyond the scope of our study.
We found good but not excellent correlations between the cross-sectional area on MRI and intraoperative hamstring graft size for all measurements in both the 1.5 and 3.0 T groups. Although Erquicia et al. [8] found that 1.5 T MRI with 4x magnification was far superior to ultrasound to predict the intraoperative size of hamstring autografts, they suggested that ultrasound should be considered an alternative to MRI because of “the high variability observed among different studies because of different MRI scanners, resolutions, and magnifications.” The benefit of ultrasound is that it is inexpensive and widely available; however, ultrasound has been shown to be less reliable in predicting the preoperative graft size, is user-dependent, and would require additional testing beyond diagnostic MRI for ACLs [25]. Although the correlations between the cross-sectional area as seen on MRI and intraoperative hamstring size in this study are not as strong as the correlation observed by Erquicia et al. [8] (r = 0.86), we found that routine preoperative MRI can still be used to reasonably predict the intraoperative hamstring autograft size. Although the measurements at the gracilis did not provide strong positive predictive values or sensitivities, those at the medial femoral condyle and those using the semitendinosus had values indicating improved predictive ability for autograft size. Using a clinical picture archiving and communication system (PACS) workstation has been shown to provide adequate measurements to reliably predict autograft size [2, 3, 29]. We speculate that this will save cost and physician time compared with performing these functions on a separate or outside measurement system.
The minimum cross-sectional area needed to produce an autograft with an 8-mm diameter was 19.9 mm2. The three measurements that showed the strongest correlation with intraoperative autograft size were the same for both the 1.5 T and 3 T MRI groups. Our cutoff value for the minimum cross-sectional area of the semitendinosus at the medial femoral condyle (12.3 mm2 for 1.5 T and 11.4 mm2 for 3 T) was similar to the cutoff value found by Serino et al. [29] (13.2 mm2). Our mean semitendinosus and gracilis measurements were 18.8 mm2 and 17.5 mm2 for the 1.5 T and 3 T groups, respectively. Erquicia et al. [8] found a cutoff of 25.5 mm2, while Leiter et al. [19] used the upper 95% CI for patients with a 7.5-mm graft to find a minimum semitendinosus and gracilis cross-sectional area of 14.5 mm2. This variability may be because of different MRI measurement techniques (as mentioned earlier), different calculation methods, and differences in sample size and gender ratios between these study populations.
Conclusion
In this study, we showed that a routine preoperative acute knee injury MRI can accurately predict if a patient will have a hamstring autograft of sufficient size to allow for ACL reconstruction. In clinical practice, this can assist orthopaedic surgeons in graft selection and preoperative planning. For both 1.5 T and 3 T MRI, the semitendinosus at the medial femoral condyle provides the best measurements to reasonably predict the size of double-bundle autografts. A combined cross-sectional area of at least 19.9 mm2 was sufficient to produce an 8-mm graft. Given the findings thus far, larger studies can be performed to assess prospectively the ability of MRI in predicting graft size. Since orthopaedic surgeons may sometimes need to evaluate hamstring tendons directly, it would be valuable to determine how well surgeons can use preoperative MRI to predict autograft size. Finally, since only four-strand autografts were used in this study, it would be useful to assess whether MRI prediction is adequate for multistrand grafts as well [28].
Acknowledgments
We thank Allison Tegge PhD, of the Fralin Biomedical Research Institute at Virginia Tech Carilion, for her statistical support and instruction. We also thank Christopher K. John MD of the Carilion Clinic Institute of Orthopaedics and Neurosciences for contributing surgical cases to this study.
Footnotes
Each of the authors certify that neither he or she, nor any member of his or her immediate family, has no commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.
Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA approval status, of any drug or device before clinical use.
Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.
This work was performed at Virginia Tech Carilion School of Medicine, Roanoke, VA, USA.
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