Significant Variation in Quadriceps Tendon Thickness Is Observed Among Cadaveric Specimens

Abstract

Purpose

To evaluate variation in quadriceps tendon (QT) morphology in cadaveric specimens, focusing on tendon thickness, and to improve current QT harvesting techniques and graft preparation for anterior cruciate ligament (ACL) reconstruction.

Methods

Eight fresh-frozen, skeletally mature, midthigh-to-toe cadaveric legs with no history of knee surgery and at least 65 mm of tendon were utilized. There were 4 males and 4 females. The average donor age was 80.4 years (range, 69-93 years), and the average height was 66.1 inches (range, 59 to 72 inches). The tendon was exposed with an open extensile approach via a longitudinal midline incision. Tendon thickness was measured using calipers at the superior pole of the patella and 65 mm proximal to the insertion. Univariate analysis was used to generate sample means, standard deviations, and 95% confidence intervals (CIs).

Results

Tendon thickness at the distal patellar insertion site was a mean (standard deviation [SD]) of 8.7 (1.4) mm. At 65 mm proximal to the patellar insertion site, the mean (SD) thickness of the tendon was 6.5 (1.1) mm. The 95% CIs were 7.7 to 9.7 mm for the distal insertion and 5.8 to 7.2 mm for the proximal tendon. The 2 measurement sites had a mean (SD) difference of 2.2 (0.7) mm (95% CI, 1.6-2.8 mm; P < .001). Overall, the proximal tendon showed a reduction in thickness by a mean (SD) of 25.3% (6.7%) compared to the thickness at the patellar pole.

Conclusions

Quadriceps tendons had a significant decrease in thickness, measuring distally to proximally. However, the thickness was not uniform, and the variation was high among specimens tested.

Clinical Relevance

Anatomic variations in QT thickness may contribute to variable graft diameters in ACL reconstruction, highlighting the importance of accounting for tendon morphology for graft preparation, ACL tunnel position, and size.

Anterior cruciate ligament injuries are among the most common knee injuries, occurring at a rate of approximately 150,000 times per year in the United States.¹ Common autograft options in anterior cruciate ligament reconstruction (ACLR) surgery include bone–patellar tendon–bone (BTB), hamstring (HS), and quadriceps tendon (QT). In current literature, autografts are preferred over allografts of ACLR in active younger patients due to higher patient-reported outcomes, lower rates of failure, and improved sports function.2, 3, 4, 5, 6, 7, 8

There is variation in the literature on the superior graft choice for ACLR, but BTB has long been considered the gold standard. QT autograft use has gained popularity given improved harvesting and fixation techniques, superior graft biomechanics compared to native BTB, and decreased harvest site morbidity compared to other grafts.9, 10, 11, 12, 13, 14, 15, 16 The BTB autograft is associated with patella fracture and anterior knee pain, while the HS autograft is associated with hamstring weakness, injury to the saphenous nerve, and relatively increased revision rates.11, 12, 13, 14, 15, 16, 17 While the QT autograft has many advantages, it has also been associated with possible complications, including quadriceps weakness, extensor lag, arthrofibrosis, and postoperative hematomas.^17,18 Despite these drawbacks, the combination of modern graft harvesting techniques and similar outcomes has led to an increase in QT utilization since 2014 among surgeons globally.^9,10 This increase in QT utilization for ACLR necessitates standard guidelines for effective graft harvesting and precise dimension prediction methods.

Several modalities have been studied for potential predictors of QT and graft thickness. Strong positive correlations (r > 0.7) have been shown between predicted QT length with magnetic resonance imaging (MRI) and QT length under direct intraoperative observation, with MRI offering a resolution for QT thickness of 0.2 mm.^19,20 Additionally, ultrasound has been used for dimensional predictive studies, with some studies showing its superiority to MRI.21, 22, 23 Individual studies have established medium-strength correlations between QT length and height, age in pediatric populations, and weight.^19,23, 24, 25, 26, 27 It is important to note that although radiological studies may be accurate in predicting graft thickness, intraoperative complications can negatively affect graft size. For example, HS tendon harvesting is prone to premature tendon amputation and small graft diameters due to possible technical errors.28, 29, 30 Therefore, surgeons should consider both the intrinsic dimensions of the tendon and the difficulty of the graft harvest when considering patient-specific graft preparation.

Given that QT length and thickness are correlated but highly variable between patients, tendon length is an important consideration when tailoring graft harvests to patient-specific measurements for harvesting appropriate graft dimensions. Improper graft lengths can result in many complications, including graft extrusion, fixation failure, and undesired graft biomechanics.31, 32, 33 Takeuchi et al.²¹ state that harvesting QT with a fixed width may result in autografts that are thinner proximally than they are distally. They reported a range of mean QT thickness from 1.9 to 7.1 mm and that the proximal end may be thinner over 65 mm proximal to the patellar insertion.

Despite these documented risks with fixed-width harvesting, there are little data regarding natural QT variance among patients with relative measures of dispersion. Anatomic variation may pose challenges in ensuring reproducible graft length and width. Anatomic variance, surgeon exposure, and subtle changes in harvest technique may compromise an ideal QT graft by potentially affecting size, fixation, and healing within the femoral and tibial tunnels. In some cases, smaller grafts have been associated with increased susceptibility to failure, as seen in HS autograft literature, emphasizing the importance of considering tendon dimensions during harvest.^28,29

The purpose of this study is to evaluate variation in QT morphology in cadaveric specimens, focusing on tendon thickness, to improve current QT harvesting techniques and graft preparation for ACLR. We hypothesize that our study will show tendinous thickening as the tendon traverses distally and also find a statistically significant difference between proximal and distal tendon thicknesses.

Methods

Specimen Selection and Demographics

A total of eight fresh-frozen, skeletally mature, midthigh-to-toe cadaveric legs with no history of surgery were utilized for this study. There were 4 males and 4 females. The average donor age was 80.4 years (range, 69-93 years), and the average height was 66.1 inches (range, 59-72 inches) (Table 1). All cadavers had a minimum of 65 mm of viable thigh proximal to the patellar insertion point of the QT. This distance from the patellar insertion was chosen as the measurement site for assessing QT thickness in accordance with recommended QT grafting techniques for a graft length averaging 65 to 70 mm.¹⁸

Table 1.

Cadaver-Specific Quadriceps Tendon Thickness Chart

Sample ID	Age, y	Sex, M/F	Height, inches	BMI	Laterality, L/R	Distal Thickness, mm	Proximal Thickness, mm	Difference, mm
1	80	F	67	15.35	R	9.6	6.4	3.2
2	73	M	72	18.70	L	9.4	6.3	3.1
3	86	M	68	17.03	R	8.7	6.7	2
4	69	M	70	21.52	L	7.7	5.6	2.1
5	74	F	59	17.37	L	6.9	4.9	2
6	85	F	63	24.27	L	8.1	6.6	1.5
7	93	M	68	21.28	R	11.5	8.8	2.7
8	83	F	62	21.58	L	7.6	6.5	1.1
Mean	80.4	N/A	66.1	19.64	N/A	8.7	6.5	2.2
σ	7.4	N/A	4.1	2.80	N/A	1.4	1.1	0.7

BMI, body mass index; L, left; N/A, not applicable; R, right.

Tendon Measurement

The QT was visualized with an open extensile approach with a midline incision. The QT was separated with a scalpel and blunt dissection from the subcutaneous layer of the skin, and a marker was used to mark the superior pole of the patella. Digital calipers were used to measure the distance from the mark to 65 mm proximal on the tendon, using the orientation of the tendinous fibers to provide a reference for parallel measurement (Fig 1). After the fascia was removed, the posterior aspect of the QT was freed medially. The vastus medialis obliques and underlying intra-articular synovium were released with full dissection, taking careful measures to not violate the superior aspect of the patella. The freed tendon was then measured once with calipers at both the mark on the superior pole of the patella and the mark 65 mm proximal to it (Figs 2, 3). Thickness measurements were recorded to the nearest tenth of a millimeter. All thickness and length measurements were performed by a board-certified orthopaedic surgeon (A.S.) with each measurement individually confirmed by four study team members (M.S., A.V., J.W., Y.G.).

Fig 1.

Marking the patellar insertion point and marking 65 mm proximal to it.

Fig 2.

Measuring the distal thickness of the quadriceps tendon at the patellar insertion point.

Fig 3.

Measuring the proximal thickness of the quadriceps tendon 65 mm proximal to the patellar insertion point.

Statistical Analysis

An a priori power analysis was conducted to determine the minimum sample size required for the study. Results indicated the required sample size to achieve 80% power for detecting a large effect at a significance criterion of α = 0.05 was n = 8 for a 1-tailed dependent Student t test. Analysis was performed using basic univariate statistics to generate the sample mean, standard deviation, range, 95% confidence interval (CI), and coefficients of variation (CVs) of the data set. The thickness difference between the 2 measurement points was calculated with sample means to quantify the change between points of the tendon harvest path, and a paired t test was used to compare thickness at the insertion site and proximally. Calculations were performed using Visual Basic for Applications software.

Results

The distal patellar insertion site measurements had a mean (standard deviation [SD]) thickness of 8.7 (1.4) mm (range, 6.9-11.5 mm). The measurement site 65 mm proximal to the patellar insertion point showed a mean (SD) thickness of 6.5 (1.1) mm (range, 4.9-8.8 mm). The SDs of thickness for the distal and the proximal measurement sites of the QT were 15.8% and 16.2%, respectively, compared to the total tendon thicknesses. The CV was 17.27% (95% CI, 11.42%-35.15%) at the proximal measurement site and 16.82% (95% CI, 11.12%-34.24%) at the distal site, indicating substantial variability across thickness values at both locations. The 2 measurement sites had a mean (SD) difference of 2.2 (0.7) mm. The 95% CI for the mean of each measurement location ranged from 7.7 to 9.7 mm for the distal tendon insertion and 5.8 to 7.2 mm for the proximal measurement point. A statistically significant difference (P < .001) was observed between the 2 regions when compared using a paired t test, showing a difference of 2.2 mm (95% CI, 1.6-2.8 mm). Overall, the tendon reduced in thickness by a mean (SD) of 25.3% (6.7%) of the maximal insertional thickness of the tendon (Table 1, Fig 4).

Fig 4.

Quadriceps tendon measurement location versus measurement thickness with 95% confidence intervals.

Discussion

The current study shows a significant increase in tendon thickness from the patellar insertion point to the proximal aspect of the tendon. Additionally, specimens show substantial variation in tendon thickness, as evidenced by CV values of over 10% for both measurement sites. The current study’s finding of variation in QT thickness is important for surgeons to consider in the context of ACLR graft harvesting.

Similar to MRI studies involving hamstrings, the QT length and thickness have been reported in radiological studies. A 1999 study by Staeubli et al.³⁴ measured several parameters of QT dimensions, including average measurements of QT thickness at the central, most proximal, and most distal points of the tendon with magnetic resonance arthrography. The authors found the anterior-to-posterior distance of the QT at all 3 points to be a mean (SD) of 8 (1) mm for men and 7 (1) mm for women.³⁴ In addition, it was noted that the linear measurement of the QT insertion on the patellar base is a mean (SD) of 18 (3) mm in men and 16 (2) mm in women. Another study by Yamasaki et al.¹⁹ found that the mean (SD) QT thickness recorded with MRI in a population that included pediatric patients was 6.3 (0.2) mm. Both of these studies show the considerable variability in QT thickness at varying points of the tendon.

While previous studies report absolute tendon thickness, our study additionally quantifies its relative variability using the CV, adding a reproducible and methodologically consistent dimension to the existing literature. Incorporating CV values is particularly relevant for ACLR graft selection, as they highlight the degree of dispersion in tendon thickness among individuals, which may impact graft reliability and consistency. Additionally, while prior MRI-based studies have reported average QT thickness values, they have not specifically quantified the extent of natural variation across different patient populations.

Although QT thickness has been extensively described with radiography, there is limited evidence provided from cadaveric anatomic studies and little quantification of natural variation among populations. Several studies have included measurements serving as a foundation for our current study. Harris et al.³⁵ reported a QT thickness of 7 mm (range, 6.4-7.8 mm), while Urchek et al.³⁶ reported that the tendon’s average thickness is between 7 and 8 mm. Strauss et al.³⁷ conducted a cadaveric study measuring 18 nonpaired specimens with a 3-dimensional coordinate measuring system. They found the median QT length to be 86.9 mm and the median thickness of 60 mm proximal to the patellar insertion to be 7.5 mm. A more extensive study measured tendons intraoperatively to find that the range of QT thickness at the tendon midpoint varied from 4 to 10 mm.²⁴ The smallest average QT thickness was reported to be as little as 1.9 mm.²¹

The findings of our study for average QT thickness were similar to the above-cited studies. More notably, the current study validated the varied range of QT thickness between specimens, as shown in the study by Strauss et al.³⁷ The range for the proximal measurement site in our study was between 4.9 and 8.8 mm, while Strauss et al.³⁷ showed a range of 4.6 to 9.6 mm, corroborating our findings despite methodological differences in measurement. Our study further quantifies the variation among QT by showing a CVs between measurements sites that are well above 10%, a measure of natural sample variation that is underreported in the literature.

To address challenges in graft selection, several graft harvesting techniques have been created to accommodate patients with insufficient native tendon dimensions or those with a predisposition to graft-harvesting complications. One alternative for patients who do not offer a sufficiently thick QT for lengths of over 60 mm is to use a patellar bone plug.¹⁹ However, bone plug use has been associated with high rates of intraoperative and short-term follow-up postoperative fractures.³⁸ In the case of inadequately thick proximal QTs, 1 technique to avoid thin grafts is harvesting a longer graft length and subsequently backfolding and suturing the proximal end of the graft to increase the thickness of the proximal aspect. Several studies have confirmed that most patients have a tendon length greater than 65 mm, which would make this approach feasible on most patients.^20,21,23 With the results from the current study, we emphasize that even as proximal as 65 mm, there is a significant variation of QT thickness that necessitates utmost attention when harvesting tendons for graft creation. This understanding should aid preoperative planning and intraoperative decision-making.

Further investigation is needed to determine the extent that anatomic variation and the potential for native tendon insufficiency have a role in clinical outcomes.

Limitations

The primary limitation was the small sample size of 8 cadavers. This gives the current study limited power in our calculated means of each measurement. The external validity of this study is challenged by the fact that tissue measurements may differ in vivo and that graft dimensions were not studied from our cadaveric specimens; tendon harvests may yield grafts of differing lengths. Additionally, the age range of our cadaveric specimens was older than the typical population undergoing ACLR (often younger athletes). While the cadaveric patient population in this study is older, age has only been correlated with QT thickness variations among pediatric populations; QT autografts for ACLR are viable and efficacious options for geriatric patients.^20,23,39,40

Conclusions

Quadriceps tendons have a significant decrease in thickness from distal to proximal. However, the thickness was not uniform, and the variation was high among specimens tested.

Disclosures

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: T.C. is a consultant or advisor for Arthrex. A.S. is a consultant or advisor for Arthrex and has equity or stocks with Medacta USA. All other authors (M.S., A.V., J.W., Y.G., C.C.) declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.