Prediction of the Graft Size of 4-Stranded Semitendinosus Tendon and 4-Stranded Gracilis Tendon for Anterior Cruciate Ligament Reconstruction

Abstract

Background: Little information is available regarding parameters that would enable a surgeon to predict the size and length of 4-stranded semitendinosus (ST) and 4-stranded gracilis tendon (GT) grafts.

Purpose: To evaluate whether certain preoperative anthropometric data enable prediction of the size of 4-stranded ST and GT autograft for anterior cruciate ligament (ACL) reconstruction.

Study Design: Cohort study (prevalence); Level of evidence, 2.

Methods: This study involved 235 Chinese Han patients who underwent double-bundle ACL reconstruction with 4-stranded autogenous ST and GT grafts. Preoperatively, the authors recorded height, weight, body mass index (BMI), gender, age, and sports activity. During surgery, the usable length of the ST and GT and the diameter of the 4-stranded grafts made of ST and GT were measured. Multiple regression analysis was used to determine relationships between anthropometric measurements and the length and diameter of intraoperatively measured ST and GT grafts.

Results: Strongest correlations for ST and GT length and GT graft diameter were height and weight. The strongest correlations for ST graft diameter were gender and weight. Body mass index had only moderate correlations with ST and GT graft size. Self-reported activity level was not correlated. Women had significantly smaller GT and ST graft diameters and shorter tendon lengths than did men. Semitendinosus graft size was significantly larger and longer than was the GT graft (7.4 ± 0.7 mm vs 5.9 ± 0.6 mm and 279.9 ± 20.8 mm vs 251.5 ± 20.8 mm, respectively). Simple regression analysis demonstrated that height, weight, and BMI can be used to predict ST and GT autograft length and diameter.

Conclusion: Several preoperative anthropometric measurements showed correlation with the length of the ST and GT and diameter of 4-stranded ST and GT grafts. The current data may provide surgeons with important preoperative information about size of ST and GT grafts and would be useful for patient counseling and alternative graft source planning.

Keywords

semitendinosus tendon (ST)gracilis tendon (GT)autograft anthropometric correlation size

Anterior cruciate ligament (ACL) reconstruction with hamstring tendons has gained widespread popularity.^10,15 Clinical studies have revealed that autogenous semitendinosus (ST) and gracilis tendon (GT) grafts provide equal postoperative results but with decreased risk of anterior knee pain due to donor-site morbidity when compared with the bone–patellar tendon–bone autograft.^1,2,4,5,11 In addition, improved fixation techniques have led to increased use of ST and GT as an alternative for the bone–patellar tendon–bone graft.^1,4

Diameters and lengths of autogenous ST and GT show clinically significant anatomic variation.^3,8,9 The difficulty in predicting the available diameters and lengths of these tendons may affect their use as ACL grafts. In addition, common fixation techniques require grafts of a certain diameter or length. Thus, it might be beneficial for the surgeon to be able to predict the size of a potential ST or GT autograft to avoid the use of a graft of an insufficient size or length. If predictive factors indicated that the graft might be of insufficient size or length, alternative graft sources could be procured before surgery.

To our knowledge, only a few studies have found a correlation between simple anthropometric measurements and size of the hamstring graft.^{6,12,14,16,17} However, in previous studies, the graft was made of 2 strands of ST and 2 strands of GT for single-bundle ACL reconstruction. No data are available regarding the grafts made of 4 strands of ST and 4 strands of GT, respectively, for double-bundle ACL reconstruction.

The purpose of this study was to evaluate whether preoperative measurements such as height, weight, body mass index (BMI), gender, age, and sports activity enable prediction of the size of 4-stranded ST and 4-stranded GT autograft. We hypothesized that preoperative anthropometric data would be useful for the prediction of the size of the ST and GT graft for double-bundle ACL reconstruction.

Methods

This was a retrospective study of data prospectively collected at the time of surgery. This study was approved by the medical ethics committee and the review board of our institution. All patients signed informed consent before the study. From December 2009 to March 2011, 235 Chinese Han subjects who had undergone double-bundle ACL reconstruction with 4 strands of ST and 4 strands of GT were enrolled in the study (167 male and 68 female patients). Preoperatively, we recorded height, weight, BMI, gender, age, and sports activity.

All graft harvest procedures were performed by an experienced orthopaedic surgeon, as reported by Zhao et al.¹⁹ Briefly, a 2- to 3-cm-long longitudinal incision was made at the medial side of the tibial tubercle and the distal insertion sites of the ST and GT. The distal ends of the ST and GT were peeled off with a 2- to 4-cm-long periosteum flap to increase the total length of the tendons, and the tendons were removed with a tendon stripper. After scraping off the muscle, the tendon was truncated at a site where the tendon volume was about 50% of its thickest part. The length of the tendon was measured as its usable length, and the length of the 4-stranded graft was calculated. Both ends of the tendon were sutured with No. 2 polyester sutures. The tendon was first folded, and two No. 2 polyester sutures were first passed through this first loop. Then, the tendon was folded again, and a polyester tape was passed through the second loop, to make a 4-stranded graft (Figure 1). The polyester tape was used for graft size measurement and graft fixation.

The diameter of the ST and GT graft was measured with nonslotted tubes sizing from 5 to 10 mm, with 0.5-mm step increase. The measurements were done by the same group of surgeons, according to the same principle. The smallest sizing cylinder through which the proximal end of the graft could be pulled with maximal manual force was considered the diameter of the proximal end of the graft. The smallest sizing cylinder through which the whole graft could be pulled with maximal manual force was considered the diameter of the distal end of the graft. The distal end of the graft was always 1 to 2 mm larger than was the proximal end because of the tendon size increase and the in-braided sutures, and the measurement result was not evaluated in this study. The measurement result of the proximal end of the graft was used to represent the size of the whole graft and analyzed in this study.

In our clinical practice, we always use the ST graft to reconstruct the anteromedial (AM) bundle. The acceptable lengths of the ST graft in the femoral and tibial tunnels are both at least 18 mm, and the graft length within the joint is always less than 24 mm. Therefore, we felt 6 cm to be satisfactory for an ST graft. The GT graft is always used to reconstruct the posterolateral (PL) bundle. The acceptable lengths of the GT graft in the femoral and tibial tunnels are both at least 18 mm, and the graft length within the joint is always less than 19 mm. Therefore, we felt 5.5 cm to be satisfactory for a GT graft. In our opinion, a 4-stranded ST graft with diameter of less than 6 mm and a 4-stranded GT graft with diameter less than 5.5 mm were considered too small. There is no consensus in the literature regarding what graft size is too small for ACL reconstruction. In our clinical practice, we arbitrarily consider a less than 5.5-mm GT graft and a less than 6-mm ST graft inadequate for PL and AM ACL bundle reconstruction because we have found a higher failure rate in patients with these smaller grafts than with larger grafts.

Statistical Analysis

Independent-samples t tests were used to identify gender differences between clinical and intraoperative variables. Correlation coefficients (Pearson r) and simple linear regression were used to determine the relationship between the outcome variables (diameter of ST and GT, length of ST and GT) and the predictor variables (age, gender, height, weight, and BMI). Differences were considered significant when P values were below .05. All statistical analysis was done using SPSS program version 17.0 (SPSS Inc, Chicago, Illinois).

Results

The mean age of the patients was 28.1 ± 10.0 years (range, 13-61 years). The mean measurements were as follows: weight (71.0 ± 13.7 kg; 156.5 ± 30.2 lb), height (171.9 ± 7.9 cm; 5.6 ± 0.3 ft), BMI (23.9 ± 3.5), and Tegner score (6.15 ± 0.8).

The ST graft size was significantly larger and longer than was the GT graft (7.4 ± 0.7 mm vs 5.9 ± 0.6 mm and 279.9 ± 20.8 mm vs 251.5 ± 20.8 mm, respectively). The ST and GT graft diameters with percentage frequency are shown in Table 1. The averaged measurement results are shown in Table 2. Compared with male subjects, female subjects were significantly shorter and lighter and had lower BMIs, smaller ST and GT diameter, and shorter ST and GT length.

Table 1

Size Distribution of the 4-Stranded Grafts^a

	Diameter, mm
Graft	5	5.5	6	6.5	7	7.5	8	8.5	9
ST
In sum	0	2 (0.9)	10 (4.3)	32 (13.6)	55 (23.4)	66 (28.1)	47 (20)	18 (7.6)	5 (2.1)
In males	0	0	0	13 (7.8)	34 (20.4)	55 (32.9)	42 (25.1)	18 (10.8)	5 (3)
In females	0	2 (2.9)	10 (14.7)	19 (27.9)	21 (30.9)	11 (16.2)	5 (7.4)	0	0
GT
In sum	21 (8.9)	78 (33.2)	75 (32.0)	42 (17.9)	16 (6.8)	2 (0.8)	0	1 (0.4)	0
In males	13 (7.8)	45 (26.9)	60 (35.9)	35 (21.0)	12 (7.2)	1 (0.6)	0	1 (0.6)	0
In females	8 (11.8)	33 (48.5)	15 (22.0)	7 (10.3)	4 (5.9)	1 (1.5)	0	0	0

Data are presented as n (%). GT, gracilis tendon; ST, semitendinosus tendon.

Table 2

Gender Differences^a

	Males	Females	t	P
Age, y	27.25 ± 8.4	29.88 ± 13.1	1.83	.0690
Height, cm	175.0 ± 6.3	164.3 ± 6.3	−11.79	<.0001
Weight, kg	75.03 ± 13.2	61.11 ± 9.7	−7.88	<.0001
BMI	24.41 ± 3.5	22.64 ± 3.4	−3.55	<.0001
ST diameter	7.59 ± 0.6	6.82 ± 0.6	−8.95	<.0001
GT diameter	5.99 ± 0.6	5.77 ± 0.5	2.66	.0083
ST length	284.67 ± 19.6	268.17 ± 18.8	−5.91	<.0001
GT length	253.4 ± 21.0	246.7 ± 19.5	−2.26	.0250
Tegner score	6.22 ± 0.9	6.12 ± 0.8	0.77	.4410

Data are presented as mean ± standard deviation. BMI, body mass index; GT, gracilis tendon; ST, semitendinosus tendon.

Correlation analysis indicated that shorter, lighter weight, and lower BMI women tended to have smaller ST and GT autograft diameters and shorter graft length. In addition, age had a weak correlation with GT autograft diameter and length (r = −0.17 and −0.14, respectively). The correlation coefficients are shown in Table 3.

Table 3

Correlation Coefficients for Relationships Between Graft Size and Length and Clinical Data^a

	ST Diameter	GT Diameter	ST Length	GT Length
Age, y	−0.054	−0.167^b	0.036	−0.14^b
Height, cm	0.410^b	0.256^b	0.60^b	0.474^b
Weight, kg	0.459^b	0.310^b	0.413^b	0.347^b
BMI	0.341^b	0.237^b	0.15^b	0.136^b
Gender	0.467^b	0.173^b	0.333^b	0.136^b
Tegner score	0.07	0.01	0.12	0.04

BMI, body mass index; GT, gracilis tendon; ST, semitendinosus tendon. The correlation between gender and other variables is calculated as a point biserial correlation (females = 1 and males = 2).

< .05.

ST Diameter

Simple linear regression showed that patient’s height and weight explained approximately 19% and 22%, respectively, of the variation in ST graft diameter. When separated by gender, we found that height, weight, and BMI were the best predictors for ST graft diameter in males, whereas height was the only important predictor for ST graft diameter in females (see Appendix 1, available in the online version of this article at http://ajs.sagepub.com/supplemental). Through regression analysis, we constructed the following predictive equations for ST diameter:

\begin{array}{l} ST Diameter = 0.78 + 0.038 (height in cm) \\ (r = 0.44; R^{2} = 0.19; P < . 001; 95 % confidence interval [CT], 6.53 - 8.18) \\ ST Diameter = 5.68 + 0.02 (weight in kg) \\ (r = 0.47; R^{2} = 0.22; P < . 001; 95 % CT, 6.75 - 8.78) \end{array}

These equations indicate that patients with a height less than 137 cm had the highest risk for having ST grafts less than 6.0 mm in diameter. Because only 2 patients (both female) had ST grafts smaller than 6 mm in diameter, the ability to predict the diameter of ST grafts of extremely small size was very limited. This was particularly true in male patients because none of the male patients had graft diameters less than 6 mm in diameter.

GT Diameter

Simple linear regression showed that patient’s height and weight explained approximately 13% and 9.2%, respectively, of the variation in GT graft diameter. When separated by gender, we found that height, weight, and BMI were the best predictors for GT graft diameter in males. There were no significant predictors for GT graft diameter in female patients (see Appendix 1, available online). Through regression analysis, we constructed the following predictive equations for GT diameter:

\begin{array}{l} GT Diameter = 1.408 + 0.026 (height in cm) \\ (r = 0.37; R^{2} = 0.13; P < . 001; 95 % CT, 5.35 - 6.48) \\ GT Diameter = 5.035 + 0.02 (weight in kg) \\ (r = 0.30; R^{2} = 0.09; P < . 001; 95 % CT, 5.59 - 6.67) \end{array}

These equations indicate that patients with a height less than 157.4 cm and weight less than 46.5 kg had the highest risk for having GT grafts less than 5.5 mm in diameter. For the male patients, a height less than 159 cm and weight less than 39 kg may predict a GT graft less than 5.5 mm in diameter. For the female patients, a height less than 157 cm may predict a GT grafts less than 5.5 mm in diameter.

We also observed that 13 males with GT graft diameters less than 5.5 mm had a mean height of 165 mm (range, 160-178 mm) and that 8 females with GT graft diameters less than 5.5 mm had a mean height of 155 mm (range, 150-158 mm).

ST Length

Simple linear regression for graft length indicated that height and weight explained approximately 37% and 17%, respectively, of variance in ST length. Height, weight, and BMI were the best predictors for ST length in males, whereas height and BMI were the best predictors for ST length in females (see Appendix 1, available online). Through regression analysis, we constructed the following predictive equations for ST length:

\begin{array}{l} ST Length = 8.7 + 1.577 (height in cm) \\ (r = 0.61; R^{2} = 0.37; P < . 001; 95 % CT, 245.3 - 313.2) \\ ST Length = 235.4 + 0.626 (weight in kg) \\ (r = 0.42; R^{2} = 0.17; P < . 001; 95 % CT, 263.6 - 316.9) \end{array}

These equations indicate that patients with a height less than 146.7 cm had a greater probability of having an ST length less than 240 mm. For male patients, a height less than 147 cm may predict an ST length less than 240 mm. For female patients, a height less than 146 cm may predict an ST length less than 240 mm.

GT Length

Simple linear regression for tendon length indicated that height and weight explained approximately 24% and 14%, respectively, of variance in GT length. Height, weight, and BMI were the best predictors of GT length in males. Height was the only important predictor for GT length in females (see Appendix 1, available online). Through regression analysis, we constructed the following predictive equations for GT length:

\begin{array}{l} GT Length = 32.2 + 1.27 (height in cm) \\ (r = 0.48; R^{2} = 0.24; P < . 001; 95 % CT, 223.5 - 278.3) \\ GT Length = 235.4 + 0.626 (weight in kg) \\ (r = 0.37; R^{2} = 0.14; P < . 001; 95 % CT, 236.8 - 284.6) \end{array}

These equations indicate that patients with a height less than 147.9 cm had a greater probability of having a GT length less than 220 mm. However, for male patients, the equation is GT Length = −23.83 + 1.584 (cm), with a 95% CI of 229.62 to 281.89 cm; for female patients, the equation is GT Length = −58.3 + 1.85 (cm), with a 95% CI of 220.1 to 281.4 cm. Less than 220 mm GT length is beyond the 95% CI for both male and female patients. Therefore, the values of height resulting from these equations to predict GT graft length less than 220 mm GT are not so reliable.

Discussion

The ST and GT grafts have become popular choices for ACL reconstruction.^1,4 Anterior cruciate ligament reconstruction generally requires triple- or 4-stranded ST and GT grafts to reconstruct the anterolateral bundle and posteromedial bundle, respectively.¹³ However, in the Chinese Han population, the size of the graft required for certain surgical techniques and procedures has not been clearly documented. Being able to predict the diameter and length of the hamstring tendon would determine whether these tendons are of adequate diameter and appropriate length before surgery.

To our knowledge, only a few studies have found a correlation between simple clinical measurements and size of the hamstring graft.^14,16,17 However, it may not be practical to use the reported equation to predict the graft size because the nationalities of the subjects studied were not clearly stated, and in our clinical practice we found there are obvious differences regarding the hamstring tendon size among people of different nationalities. The Chinese Han people account for 91.51% of the whole population of China, and China’s population accounts for 20.1% of the world population in 2010. Graft size data regarding this special nationality may be useful for surgeons treating this large group of patients.

The results of our study support our hypothesis that simple clinical measurements can effectively predict the size of ST and GT graft for double-bundle ACL reconstruction. In addition, the results in this study represent figures only applicable to the Chinese Han population, and they may not be able to be extrapolated to the world population as a whole. Height, weight, BMI, and gender are the strongest predictors for ST diameter, and all 3 measurements have moderate correlation with GT diameter. In this study, we also found a weak to moderate correlation between BMI and the ST and GT diameter. Tuman et al¹⁷ analyzed the diameter of the hamstring tendons graft of 106 patients who had ACL reconstructions, and they found a correlation between the graft diameter and height, weight, gender, and BMI. They concluded that height, as the best predictor for hamstring tendon diameter, can be used to calculate the graft size, particularly in women.

In our study, age had a weak correlation with GT diameter and length, which was similar to the findings of Schwartzberg et al.¹⁴ Tuman et al¹⁷ also reported that age had a weak correlation with hamstring graft diameter in women but not in men or with graft length.

There was no correlation between graft size and sports activity, which suggests that athletics does not affect ST and GT dimensions. These results are consistent with data from Treme et al.¹⁶ Similar results were also found for the patients of Pichler et al.⁹

These data demonstrated that older, short, and lighter females with lower BMI should be considered at highest risk for small graft diameter and short graft length. For both the entire group and gender groups, ST graft size was significantly larger and longer than were GT grafts. In a cadaveric study, Pichler et al⁹ demonstrated that there are considerable differences in the length and cross-section of the ST and GT and observed a statistically significant gender difference in diameter of ST and GT. Treme et al¹⁶ evaluated 50 patients and found that female patients presented a graft size significantly smaller and shorter than that of men.

For ST graft size, 7 mm should be considered the minimum diameter for single-bundle ACL reconstruction.^7,18 In our opinion, a 4-stranded ST graft with size of less than 6 mm and a 4-stranded GT graft with size less than 5.5 mm were considered too small for double-bundle ACL reconstruction. Our equations indicate that patients with a height less than 157.4 cm and weight less than 46.5 kg are at highest risk for having a GT graft of less than 5.5 mm in diameter, especially for the female gender.

For graft length, height is the most significant predictor for both ST and GT length, which is consistent with the findings of Treme et al.¹⁶ For 4-stranded hamstring tendon, a graft with a minimum length of 60 mm is recommended.^7,18 In our clinical practice, an ST tendon with a length less than 24 cm (to result in an ST graft less than 6 cm in length) and a GT tendon with a length less than 22 cm (to result in a GT graft less than 5.5 cm) were considered too short. (For double-bundle ACL reconstruction with two 4-stranded hamstring grafts, we always use the ST graft to reconstruct the AM bundle. The acceptable lengths of the ST graft in the femoral and tibial tunnels are both at least 18 mm, and the graft length within the joint is always less than 24 mm. The GT graft is always used to reconstruct the PL bundle. The acceptable lengths of the GT graft in the femoral and tibial tunnels are both at least 18 mm, and the graft length within the joint is always less than 19 mm.) It was observed in this study that patients with a height less than 146.7 cm and 147.3 cm and the female gender show greater probability of having an ST and GT graft with a length equal to or less than 240 mm and 220 mm, respectively.

With regard to graft length, height appears to be the best predictor. The relationship between height and GT length was not as strong as that of height and ST length, suggesting that the use of simple clinical measurements may be better suited to estimate ST length. In addition, in cases in which a quadrupled ST or GT would not be possible because of a small graft size, both the ST and GT could be combined with a paired doubled graft. We believe that the preoperative clinical measurements used in this study resulted in a substantial improvement in our ability to accurately estimate graft size parameters, and these data may be useful for graft selection in certain surgical techniques.

Some limitations of the current study should be emphasized. There exists a difference between the cylindrical sizers and diameters. To improve the accuracy of graft measurements, the diameter was measured in 0.5-mm increments instead of 1-mm increments, and all measurements were performed by the same group of physicians according to the same principle. In addition, the length and the size of the graft may be affected by the harvest technique. To some extent, the graft cut to determine the length is a bit subjective and arbitrary. To minimize this impact, all harvests were performed using a consistent technique by 1 proficient surgeon.

Conclusion

The length and diameter of ST and GT grafts for double-bundle ACL reconstruction were significantly correlated with weight, height, and BMI. The graft length and diameter among the Chinese Han population can be predicted using the simple clinical measurements. These data provide important preoperative information for the surgeon and are useful in planning graft options in the event of inadequate graft size.

Footnotes

The authors declared that they have no conflicts of interest in the authorship and publication of this contribution.

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