Radiographic Features of Acute Patellar Tendon Rupture

Abstract

Objective

The purpose of our study was to assess soft tissue features of acute patellar tendon rupture on lateral knee radiograph that would facilitate early diagnosis.

Methods

The participants were divided into two groups of 35 patients each. There were 28 men and seven women with a mean age of 46 years in the control group and 26 men and nine women with a mean age of 47 years in the rupture group. The lateral knee radiograph of each patient was evaluated for Insall–Salvati ratio for patella alta, increased density of the infrapatellar fat pad, appearance of the soft tissue margin of the patellar tendon and bony avulsions.

Results

In the rupture group there were three consistent soft tissue radiographic features in addition to patellar alta. These were increased density of infrapatellar fat pad; loss of sharp, well‐defined linear margins of the patellar tendon and angulated wavy margin of the patellar tendon while in the control group these features were not observed.

Conclusions

The soft tissue radiographic features described in the rupture group are consistent and reliable. When coupled with careful clinical assessment, these will aid in early diagnosis and further imaging will be seldom required.

Introduction

An acute patellar tendon rupture is a disabling injury. The true incidence of patellar tendon ruptures is not known but is the third most common injury to the extensor mechanism of the knee following patellar fractures and quadriceps tendon rupture. Siwek and Rao note that it usually occurs in patients younger than 40 years of age after a fall or a direct trauma1. Early diagnosis of a patellar tendon rupture is desirable, followed by prompt surgical repair to achieve a good functional outcome. In spite of its typical features, Bhargava et al. report that the patellar tendon rupture is a commonly missed injury2. The period between the injury and surgical repair is of paramount importance. Delay in diagnosis and treatment is associated with an inferior clinical outcome, as described by Matava3 and Levine,4 and in particular results in knee stiffness and a reduction in quadriceps strength. Direct surgical repair is not possible in cases that are diagnosed after considerable delay and complex surgical procedures as tendon transfers are required; hence a prompt diagnosis of this potentially disabling injury is essential. Careful clinical assessment is the key to clinching the diagnosis. A robust clinical examination is not always possible as is often limited due to pain and patient's apprehension; hence it is challenging to elicit the typical clinical features, thus contributing to missed or delayed diagnosis. Knee radiographs are performed routinely in knee injuries to exclude fractures. Lateral radiographs of the knee may show a high‐riding patella in a patellar tendon rupture. Ultrasonography and magnetic resonance imaging (MRI) are useful in equivocal cases. The most commonly described radiographic feature of acute patellar tendon rupture is patella alta on the lateral knee radiograph but soft tissue features of a patellar tendon rupture on a plain lateral knee radiograph are not very much described in the literature.

On a normal lateral radiograph of the knee, the patellar tendon has a smooth, sharp soft tissue margin which runs from the lower pole of the patella to the tibial tuberosity and the infrapatellar fat pad (IPFP) is of the same density as the surrounding soft tissue (Fig. 1). Our hypothesis was that the rupture of the patellar tendon must result in a disruption of the normal soft tissue margin of the tendon seen on the lateral radiographs and bleeding as a result of rupture must have an effect on the normal soft tissue appearance. The purpose of our study was to investigate whether an acute patellar tendon rupture has an effect on the soft tissue appearance in the infrapatellar area, to discuss the results and to study the soft tissue changes in an acute patellar tendon rupture that could increase diagnostic accuracy and aid in early diagnosis with potential benefits of less reliance on further imaging, prompt surgical repair and an excellent functional outcome.

Figure 1.

Drawing of lateral view of knee showing that the patellar tendon has a sharp and smooth margin and a normal infrapatellar fat pad.

Materials and Methods

We retrospectively reviewed case notes and radiographs of patients with a knee injury presenting to the accident and emergency department between May 2007 and December 2012. Patients with ligament injury, bony injury, previous surgery to the knee, presenting after a week of injury, quadriceps tendon injury and skeletally immature patients were excluded from the study. Patients were divided into two groups, a control group and a rupture group. The control group consisted of 35 patients. It consisted of 28 men and seven women with a mean age of 46 years, a median of 51 years (range, 25–66), and 20 right knees and 15 left knees.

The rupture group consisted of 35 patients with surgically confirmed acute patellar tendon rupture. There were 26 men and nine women with a mean age of 47 years, a median of 49 years (range, 28–65). There were 18 left and 17 right knees. Two patients suffered from asthma but had never had any steroid therapy and two patients had sickle cell trait.

All patients had anteroposterior (AP) and lateral radiographs of the injured knee taken in a supine position at the time of presentation to the hospital. A film cassette was placed underneath the knee for AP projection and on medial side of the knee for lateral projection. The distance of the focal spot to the cassette was 1m and angle of the beam was perpendicular to the cassette. Amplification was about 15%. The lateral knee radiographs were performed using a horizontal beam and with the knee with some flexion, as tolerated by the patient. The mean time to radiographs from injury was 3 days (range 1–5 days) in the control group and a mean of 1 day (range 1–2 days) in the rupture group.

The lateral radiographs of all the patients were evaluated for the following features by one of the authors without the knowledge of the diagnosis; (i) Insall–Salvati ratio for patella alta; (ii) increased density of the IPFP; (iii) appearance of soft tissue margin of the patellar tendon; and (iv) bony fragments indicating avulsion fractures.

Results

A case note review of the surgically confirmed patellar tendon rupture reveals that the rupture was complete in all the cases. In 28 patients the tendon was ruptured at the junction of the upper and lower half of the tendon and in seven patients the tendon was ruptured at the junction of the upper third and middle third of the tendon.

Insall–Salvati Ratio

The Insall–Salvati ratio was determined by dividing the length of the patellar tendon by the length of the patella. The tendon length was measured as the length of the posterior surface of the tendon from the lower pole of the patella to the insertion of the tendon in the tibia. The patellar length was the greatest length from the superior to the inferior pole of the patella (Fig. 2). A ratio of one was considered to be normal and a ratio greater than 1.2 was suggestive of patellar alta and a a ratio below 0.8 of patella baja. In the control group, all patients had an Insall–Salvati ratio between 0.8 and 1.2 (Fig. 3) while in the rupture group all patients had a ratio greater than 1.2 (Figs 4, 5).

Figure 2.

Lateral radiograph showing the measurement for the Insall–Salvati ratio. A was “Patella length”; B was “Length of patellar tendon”.

Figure 3.

Lateral radiograph from the control group showing (A) a well‐defined soft tissue margin of the patellar tendon and (B) normal density of the infrapatellar fat pad.

Figure 4.

Lateral radiograph with rupture of the patellar tendon showing (A) loss of well‐defined soft tissue margin of patellar tendon, (B) increased density of infrapatellar fat pad, (C) angulation and wavy soft tissue margin of patellar tendon, and (D) patella alta.

Figure 5.

Lateral radiograph with rupture of the patellar tendon showing (A) loss of well‐defined soft tissue margin of patellar tendon, (B) increased density of infrapatellar fat pad, (C) angulation and wavy soft tissue margin of patellar tendon, and (D) patella alta.

IPFP

The IPFP is smooth and has same density as surrounding soft tissue in a normal lateral radiographs of the knee. We observed that in the control group the IPFP was smooth and its density was the same as the surrounding tissue (Fig. 3) while in the rupture group the IFFD was quite dense and its contour was irregular when compared with the control group (Figs 4, 5).

Appearance of Soft Tissue Margin of the Patellar Tendon

In normal lateral radiographs of the knee, the soft tissue margin of the patellar tendon is sharp and well defined. In the control group, the soft margin of the patellar tendon was sharp and well defined from the lower pole of the patella to the tibial tuberosity (Fig. 2) but in the rupture group there was a loss of sharp, well‐defined soft tissue margins of the patellar tendon from the lower pole of the patella to the tibial tuberosity and its contour was wavy and angulated (Figs 4, 5). No bony avulsions were observed in our series.

Table 1 compares the findings in the two groups.

Table 1.

Findings on the lateral radiograph

Groups (n)	Insall–Salvati ratio	Increased density of IPFP (cases)	Wavy and angulated soft tissue margin of PT (cases)	Appearance of soft tissue margin of PT
Control group (35)	0.8–1.2	0	0	All had well‐defined sharp margin
Rupture group (35)	>1.2	35	35	Loss of well‐defined sharp margin in all

IPFP, infrapatellar fat pad; PT, patellar tendon.

Discussion

The diagnosis of acute patellar tendon rupture depends on a careful history, clinical examination and imaging. Clinical assessment can be challenging due to significant pain, swelling and patient's apprehension as a result of injury. Siwek and Roa note that clinical assessment can be difficult due to the inability to feel the defect in the patellar tendon because of swelling and the ability of the patient to extend the knee due to intact retinacula1. Bhargava et al. in a study of 11 patients with acute patellar tendon rupture reported that all the ruptures were missed at initial presentation2. Delay in the diagnosis of patellar tendon rupture may cause delayed treatment that can lead to a poor result. Matava and Levine have shown that delay in surgical management can lead to a poor functional outcome with a loss of full knee flexion and decreased quadriceps strength3, 4. These studies suggest that the diagnosis of a patellar tendon rupture should be made as early as possible to achieve optimum results. In equivocal cases, further imaging is needed. Ultrasound may be available in the emergency department of some hospitals but is operator‐dependent. MRI scans may not be readily available universally and are costly.

Patella alta is the most common radiological feature described. It has low intra‐observer reliability. Kadakia and Ilahi reported in their study of 36 cases that the observers agreed in determining patella alta in two‐thirds of the cases with the traditional Insall–Salvati method but in less than half of the cases when the modified ratio was used5. Grelsamer and Meadows reported in their study that half of the cases of patella alta were missed by the traditional ratio6. Seil et al. in their study of 22 knees found there was low intra‐observer reliability using the Insall–Salvati and the modified ratio and supported the use of the Blackburne–Peel ratio7. Recently, Chin and Sodl in their study of 14 patients described IPFP disruption as a radiological sign of a patellar tendon rupture and concluded that this disruption in the contour of the IPFP on routine lateral radiographs was a reliable sign of a patellar tendon rupture8.

We observed that the lateral radiographs in the control group had a linear, sharp, well‐defined and smooth soft tissue margin of the patellar tendon and with no increase in density of the IPFP. Patients in the rupture group with surgically confirmed patellar tendon rupture had increased density of IPFP and loss of the sharp, well‐defined soft tissue margin with angulated and wavy soft tissue margin of the patellar tendon in all the lateral radiographs. We believe that the increased density of the IPFP is due to edema and displacement of the IPFP by hemarthrosis. The loss of the sharp and well‐defined soft tissue margin of the patellar tendon is due to the loss of the normal tissue tension of the tendon as a result of the rupture. Our study showed patellar alta in all cases of the rupture group; however this may not be reliable, as described in previous studies5, 6.

The strength of our study is that both groups were almost similar but its limitations are the small number of patients studied, the fact that appearances may depend upon the quality of the radiograph and interobserver reliability may be uncertain. A larger prospective study is needed to look at the diagnostic accuracy of the features described. Plain radiographs are an integral part of the evaluation of an injured knee. They are cost‐effective, simple and readily available. Almost all patients with a significant knee injury fulfil the criteria for a knee radiograph, as set out by Ottawa knee rule9. Patients with an extensor mechanism injury are not an exception. A plain radiograph may show patella alta, avulsion fractures and an associated bony injury. In addition to these, it can be used to look for soft tissue features of the increased density of IPFP, an angulated and wavy soft tissue margin of the patellar tendon with the loss of sharp, well‐defined soft tissue margins, as described in our study. To our knowledge, these soft tissue features on the lateral knee radiographs have not been described in the literature. Our study shows that these soft tissue features are consistent on the lateral radiographs in patients with acute patellar tendon rupture and are reliable radiographic signs for the diagnosis of acute patellar tendon rupture. Hence we recommend that plain lateral radiographs of the knee should be carefully examined for these features in clinically suspected cases of patellar tendon injury to increase diagnostic accuracy.

We conclude that these signs are dependable and that further investigations are seldom required, thus reducing the interval between diagnosis and treatment, which is essential for the management of this potentially disabling injury.