Patellar Tendon Length is Associated with Lower Extremity Length but Not Gender

Abstract

Background

The aim of this study was to evaluate the patellar tendon length (PTL) with focus on gender differences and possible correlations with the total leg length (TLL) and the long bones of the lower extremity.

Materials and Methods

The sample involved 50 paired lower extremities from human adult cadavers. The TLL was measured between the medial malleolus and the apex of the greater trochanter. The femoral length (FL) was evaluated as the interval between the latter and the distal margin of the lateral epicondyle of the femur and the tibial length (TL) from the distal apex of the medial malleolus to the proximal border of the medial condyle of the tibia. The PTL was measured from the apex of the patella to its proximal insertion point at the tibial tuberosity.

Results

The PTL was at a mean length of 4.29 ± 0.49 cm (right side) and 4.20 ± 0.55 cm (left side) in females and 4.42 ± 0.53 cm (right) and 4.32 ± 0.55 cm (left) in males. There were no differences regarding gender (p = .412). The left PTL was significantly shorter in both sexes (p = .022). The PTL correlated positively with FL, TL, and TLL in both sexes and sides.

Conclusion

PTL correlates significantly positively with size without gender differences.

Introduction

Patellar dislocation (PD) accounts for 2–3% of all knee injuries and represents the second most common cause of knee hemarthrosis [1, 2], with a reported recurrence rate ranging from 15 to 44% following conservative treatment [3].

After primary PD, up to 55% of all patients fail to resume their pre-traumatic sports activities [4]. Various factors such as patella alta, abnormal patellar morphology, patellar tilt, condyle dysplasia, trochlea dysplasia, general hyperlaxity, increased femoral anteversion, hypoplasia of the vastus medialis, and an increased quadriceps (Q)-angle combined with lateralisation of the tibial tuberosity contribute to primary and recurrent PD [2, 5]. In the long view, these aspects may result in anterior knee pain and osteoarthritis of the patellofemoral joint (PFJ) [2, 3].

The high-risk age range for PD lies between 10 and 17 years including a 33% higher risk in females than in males [6, 7]. Furthermore, patients with anamnestic patellar instability (PI) tend to be females and have a three times higher risk for recurrent PDs [1]. This might be on one side traced back to gender differences in contact areas and pressures of the PFJ [8]. Additionally, the biomechanics of the PFJ are considerably influenced by the patellar tendon (PT) [9, 10], since, in combination with the quadriceps tendon, the PT provides a strong posterior force vector during flexion of the knee that contributes mainly to patellar stability [4]. However, up to now, the literature does not include detailed information on gender differences and correlations with lower extremity proportions regarding the patellar tendon length and information concerning potential gender-related differences might be useful for further conservative and operative treatment options of the PFJ.

Therefore, the aim of this study was to evaluate the length of the patellar tendon with regard to gender differences and possible correlations with the total leg length and the long bones of the lower extremity.

The hypothesis was that there would be gender-specific differences concerning length and dimensions as with the Q-angle, which might have an impact on future surgical interventions of the PFJ.

Materials and Methods

Specimens

The study sample involved 50 pairs of lower extremities from adult human cadavers (25 females and 25 males) embalmed using Thiel’s method, a special embalming technique which simulates stable life-like soft-tissue conditions [11]. All of the body donors were of Caucasian origin and had given their written informed consent to participate in anatomical studies during their lifetime. Specimens showing obvious signs of severe osteoarthritis, prior surgery, or pathologies in the area of interest were excluded from the study.

Dissection and Measurement

After removal of the skin and subcutaneous tissue, the total leg length (TLL), which was defined as the interval between the distal tip of the medial malleolus and the apex of the greater trochanter, was measured. The tibia length (TL) was measured from the distal apex of the medial malleolus to the proximal border of the medial condyle of the tibia. As the next step, the femoral length (FL) was evaluated as the distance between the tip of the greater trochanter and the distal margin of the lateral epicondyle of the femur. For measurement of the PT length (PTL), the patella was excised circular from the quadriceps femoris (QF) and dissected together with the patellar tendon (PT) to the latter’s proximal insertion point at the tibial tuberosity. Here, the PT was cut-off and the PTL was evaluated as the interval between the cut-off point and the apex of the patella. The Q-angle was measured between a line from the anterior superior iliac spine to the centre of the patella and a line alongside the physiological leg axis from the patella’s centre to the midpoint of the tibial tuberosity. Dissections and measurements are further illustrated in Figs. 1, 2, and 3.

Fig. 1.

Scheme depicting measurement of the femoral length (FL), tibial length (TL), and total leg length (TLL)

Fig. 2.

Dissection of the PT to the tibial tuberosity

Fig. 3.

Measurement of the PTL. Arrows marks the interval between the patellar apex and the proximal insertion point at the tibial tuberosity

Measurements were taken in centimetres and degrees and performed by two observers. As training, on ten lower extremities, the interobserver and intraobserver reliability was assessed for two repeated tibial length measurements of two observers by intraclass correlation coefficients (ICC) (two-way random model). All ICC values for interobserver and intraobserver reliability were > 0.99.

Statistical Analysis

Statistical analysis was performed using SPSS statistical software (version 24.0; IBM Corp, Armonk, NY, USA). For descriptive statistics, the mean ± standard deviation (SD), median, minimum, and maximum are presented. To investigate side and sex differences for the PTL, a repeated measurement ANOVA was conducted including sides (right/left) as within-subject effect and sex (female/male) as between-subject effect. To investigate associations between length of the PTL, femur, tibia, TLL, size of the donors, and the Q-angle, Pearson correlation coefficients and Spearman correlation coefficients (for Q-angle) were calculated separately for side and sex. An a priori power analysis was performed. According to a power of 80% and a significance level set < 0.05 with an estimated difference of 1 cm in length and 5° in angle between both sexes, the required number of specimens was n = 80.

Results

The mean age of the donors was 75.4 ± 14.4 years with a range from 31 to 95. Their mean height was 167.1 ± 14.4 8.5 cm (range 152–179). Details on all evaluated data separated for side and sex are further illustrated in Table 1.

Table 1.

Measurements of patella tendon length (PTL), femoral length (FL), tibial length (TL), total leg length (TLL) in millimetres, and Q-angle separated for side and gender, measured in degrees

	Right						Left
	n	Mean	SD	Median	Min	Max	n	Mean	SD	Median	Min	Max
PTL
F	25	4.29	0.49	4.30	3.30	5.30	25	4.20	0.55	4.30	2.80	5.00
M	25	4.42	0.53	4.40	3.30	5.40	25	4.32	0.55	4.30	3.10	5.40
FL
F	25	41.94	1.73	41.90	38.70	45.70	25	41.75	2.02	41.80	37.90	46.20
M	25	44.84	2.61	44.90	39.80	49.50	25	44.97	2.46	44.70	39.40	49.10
TL
F	25	35.21	2.26	34.80	32.00	40.10	25	35.34	2.18	34.80	32.20	39.90
M	25	38.56	2.40	38.80	33.60	42.40	25	38.47	2.51	39.00	33.50	42.70
TLL
F	25	78.59	3.68	78.00	71.90	86.70	25	78.67	4.00	77.60	71.60	87.10
M	25	85.57	4.98	86.60	74.00	93.80	25	85.56	5.08	86.00	73.60	93.90
Q-angle
F	10	11.54	1.38	11.50	10.00	14.00	10	12.25	1.23	12.25	11.00	14.00
M	10	12.00	1.49	12.00	10.00	14.00	10	11.77	1.30	12.00	10.00	14.00

N = 50 pairs of lower extremities (25 females and 25 males) except for Q-angle (N = 20, 10 females and 10 males)

The PTL was at a mean length of 4.29 ± 0.49 cm (right side) and 4.20 ± 0.55 (left side) in females and 4.42 ± 0.53 cm (right) and 4.32 ± 0.55 cm (left) in males. The statistical analysis did not show differences regarding gender (p = 0.412). Concerning side, the left PTL was statistically significantly shorter in both sexes (p = 0.022) but without clinical relevance (0.01 cm in females and 0.1 cm in males).

The PTL correlated positively with FL, TL, and TLL in both sexes and sides. The exceptions were the PTL and the TL (p = 0.140) and TLL (p = 0.069) in right female extremities and the PTL and the TL (p = 0.080) in left female extremities. Detailed results are further illustrated in Table 2.

Table 2.

Pearson correlation analysis of the patellar tendon length (PTL) with the femoral length (FL), tibial length (TL), and total leg length (TLL) in female and male patients with significance levels

	FL	p value	TL	p value	TTL	p value
PTL females
Right	.415*	0.039	0.303	0.140	.370	0.069
Left	.550**	0.004	0.357	0.080	.405*	0.045
PTL males
Right	.414*	0.040	.402*	0.046	.436*	0.030
Left	.404*	0.045	.417*	0.038	.416*	0.038

*Significance at 0.05

**Significance at 0.001

The Q-angle was measured on 20 paired lower extremities and revealed a mean value of 11.5° ± 1.4° in females and 12.0° ± 1.5° in males on the right and 12.3° ± 1.2° in females versus 11.8° ± 1.3° in males on the left side, respectively. No statistically significant correlations with the PTL, TL, and TLL were observed.

Discussion

The aim of this study was to evaluate the anatomy of the patellar tendon in detail with the main focus on gender differences and possible correlations with the total leg length and the long bones of the lower extremity.

The hypothesis was that there would be gender-specific differences concerning length and dimensions as with the Q-angle, which might have an impact on future surgical interventions of the PFJ.

We found that the patellar tendon length significantly correlates with height but independent of gender.

Our investigation is of major interest as the patellofemoral joint (PFJ) represents a complex reconstruction, whose stability is maintained by an interplay of active, passive, and static stabilisers. While these act in harmony during non-pathologic knee motion, changes in this interaction, as due to trauma or anatomical variations, may result in primary and recurrent patella dislocations [1, 6, 12]. Main stabilisers include the morphometry of the trochlear groove and the quadriceps and patellar tendons, which form a posterior force vector during knee flexion, enabling fixation of the patella in its slide bearing [4]. Furthermore, the medial patellofemoral ligament (MPFL) represents an essential stabiliser, which provides approximately 60% of lateral restraint from 0° to 30° of knee flexion [6, 13]. Anatomic factors that contribute to patellar instability include an increased Q-angle and tibial tuberosity–trochlear groove (TT–TG) distance as well as trochlear dysplasia [14, 15]. Additionally, patella engagement has been stated to be associated with PD [16] and injury to the MPFL has been described in the majority of all cases of PD [6, 12]. Furthermore, knees including patella alta show reduced contact areas concerning the PFJ, which lead to increased patellofemoral stresses that facilitate PDs [4].

PDs and pathologies of the PFJ in total concern more commonly the female sex [3, 6], which has been a current topic in the literature. Balcarek and colleagues [6] compared knee magnetic resonance images (MRI) obtained from 100 patients with PD to images from 157 control patients. Authors found a statistically significantly more prominent trochlear dysplasia and TT–TG distance in females with PD.

Furthermore, the biomechanics of the PFJ are mainly influenced by the PT, which also contributes to the patellar stability. Neyret et al. [17] compared the PTL of a sample of 42 knees with a history of PD to 51 control patients using X-ray and MRI. Here, the PT was significantly longer in patients following prior PD (X-ray: 53 mm; MRI: 52 mm), when compared to the control knees (X-ray: 46 mm; MRI: 44 mm).

In our sample, the PTL was at a mean length of 4.29 cm (right side) and 4.20 (left side) in females and 4.42 cm (right) and 4.32 cm (left) in males. These values are comparable to the results of Andrikoula et al. [18], who reported a mean PTL of 4.3 cm following dissection of ten fresh-frozen cadavers. Basso and colleagues [19] evaluated a mean PTL of 6.4 cm from 22 specimens; however, authors measured from the patellar apex to the distal insertion point at the tibia.

Concerning the current sample, the statistical analysis did not show differences regarding gender (p = 0.412). Yoo and colleagues [20] reported the PTL as measured in a sample of 172 knees on sagittal and axial MRIs. Authors found a statistically significant difference between the sexes (males: 40.7 mm; females: 38.0 mm). However, the sample was imbalanced with regard to gender as there were 142 knees from male patients and only 30 from females. In addition, our study evaluates anatomic specimens and is, therefore, less biased as an MRI investigation.

In our sample, the PTL correlated positively with FL, TL, and TLL in both sexes and sides with exception of the TL and TLL in right female extremities, and the PTL and the TL in left female extremities. In comparison, in Yoo et al. [20], the PTL correlated poorly with anthropometric measures such as weight, height, and BMI.

We found no statistically significant correlations of the Q-angle with the PTL, TL, and height for both sexes. This is in line with Grelsamer et al. [21]. However, the Q-angle was only observed in 20 paired extremities.

Modern surgical reconstructions aim for individual treatment options according to the patients’ anatomy [7]. We believe that our findings indicate the necessity for individual surgery concerning the patients’ body type and height, respectively, but there is no need to specifically address the patients’ gender.

We want to outline the following limitations of our work: As our donors are mainly Caucasian females and males from Styria, there might be a potential selection bias, as our findings might not be in line with patients from other regions of this world. However, we want to underline the important benefit that this is the first study, which investigated this condition in a large series of 100 specimens, which were prepared according to a widely renowned technique by Thiel [11].

In conclusion, we found no relevant differences with respect to female or male patellar tendon lengths. The left PTL was statistically significantly shorter in both sexes without clinical relevance. The tendon correlated positively with FL, TL, and TLL with exception of the TL in right and left female limbs and the TLL in right female extremities. The authors believe that surgical interventions regarding PFJ instabilities should not be altered due to gender but solely be modified concerning patients’ TLL.

Author contributions

GMH: protocol development; data collection; manuscript writing. MD: protocol development; data collection; manuscript editing. HK: data collection; manuscript editing. GG: data collection; manuscript editing. RR: data analysis; manuscript editing. AS: data collection; manuscript editing. IV: data collection; manuscript editing. HW: data analysis; manuscript editing. PS: protocol development; manuscript writing and editing.

Funding

None.

Compliance with Ethical Standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical standard statement

This article does not contain any studies with human or animal subjects performed by the any of the authors.

Informed consent

For this type of study informed consent is not requied.