Thompson Test

Original Editor - Heather Paulis, Lauren Tréhout as part of the Vrije Universiteit Brussel's Evidence-based Practice project

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Definition/Description

The Thompson test examines the integrity of the Achilles tendon by squeezing the calf. Doctors perform it as a clinical test to identify the presence of a complete Achilles rupture.

Clinically relevant anatomy

Image: Gastrocnemius muscle (highlighted in green)

The calf musculature is made up of 3 muscles which together known as triceps surae:

• Gastrocnemius Muscle, which arises from two heads on the posterior surface of the femur, directly above the two condyles of the femur
• Soleus Muscle, which comes from an inverted U-shaped origin from the posterior aspect of the Tibia, interosseous membrane and the head and upper 1/3 of the fibula
• Plantaris Muscle, which arises from the distal lateral portion of the linea aspera. It is considered as an accessory muscle and is absent in 7–20% of limbs.

Gastrocnemius and soleus muscle blend together in the distal portion of the calf and form the tendon of Achilles. The plantaris tendon, when present, lies alongside the heel cord or is somewhat removed from it in its insertion into the medial aspect of the calcaneus.

Tendons are strong, tough bands of inelastic fibrous connective tissue that connect muscle to bone. They consist of elongated cells, minimal ground substance, and collagen fibers. Collagen fibers tightly align parallel to the direction of force. Each collagen fibril is arranged into fascicles, which contain blood vessels and nerve fibers.

Purpose

The goal of this test is to find out if there is a complete tear present in the Achilles tendon.

Technique

The patient lies prone with his foot over the end of the table. Alternatively, the patient could lie prone with his knee flexed to 90°. The examiner squeezes the calf muscles, specifically the gastrocnemius - soleus complex, with his hand. Squeezing the calf should cause contraction of the Achilles tendon, resulting in plantar flexion. If the Achilles tendon is completely ruptured, there will not be any apparent plantar flexion.

Thompson Test video provided by Clinically Relevant

The plantaris and deep toe flexors can still provide plantar flexion, even if the Achilles tendon is ruptured. This means that plantar flexion observed during the Thompson test does not entirely rule out an Achilles tendon rupture. Therefore, additional clinical signs and diagnostic methods should be employed to confirm the diagnosis. To be sure that the patient has a complete Achilles tendon rupture, there are three additional clinical signs that may be observed to corroborate the diagnosis:

1. On careful inspection, with the patient prone and both ankles fully relaxed, the foot on the ruptured side hangs straight down due to the absence of the tendon tone
2. There may be a palpable gap in the tendon, approximately 3-6cm proximal to the insertion into the calcaneus
3. There is a significant reduction in the strength of the plantar flexion.

Key Research

Simmonds described in 1957 how squeezing the calf muscles could test the rupture of the Achilles tendon. Thompson however, had noted this effect in 1955. The Simmonds-Thompson test is an indication of a possible complete rupture, but its mechanism is not completely clear. Recent studies have supported the use of the Thompson test while also highlighting the benefit of complementary diagnostic methods. Despite advancements in imaging technology, the Thompson test remains a crucial part of the physical examination for suspected Achilles tendon ruptures. It is particularly valuable in primary care and emergency settings where immediate imaging may not be available.

Sensitivity and Specificity

The Thompson test consistently demonstrates sensitivities above 0.90, making it one of the most sensitive clinical tests for identifying Achilles tendon ruptures.

A systematic review and meta-analysis published by Wang et al in 2013 found that the Thompson test had a pooled sensitivity of 0.96 and specificity of 0.93 for diagnosing Achilles tendon ruptures. This indicates that the test is highly accurate in identifying both true positives and true negatives.

Limitations

While highly sensitive, the Thompson test may not be as effective in cases of partial ruptures or older injuries. In these situations, the test may still show some plantar flexion movement, which could result in false negatives.

Imaging as a Complementary Tool

While the Thompson Test is effective in diagnosing Achilles tendon ruptures, imaging techniques such as ultrasound and MRI are essential for confirming the diagnosis and planning treatment. Ultrasound, in particular, has been shown to have a sensitivity of 95% and a specificity of 99%, providing dynamic imaging that is valuable for assessing tendon integrity and guiding management.

Mechanisms of the Thompson Test

Thompson and Dohertyfound that the squeezing test gave a positive result when the m. soleus alone was divided. To elucidate these findings, Scott and colleagues separated the m. gastrocnemius tendon from the m. soleus down to 10 cm above the Achilles tendon, the place where the fibers form the tendon. By squeezing a healthy calf, the m. soleus deformed, which made the overlying m. gastrocnemius tendon bow away from the tibia, resulting in plantar flexion. The m. soleus did not move in a longitudinal manner, while the m. gastrocnemius muscle moved about 1 cm proximally. These findings confirm the ultrasound findings. When the squeezing test was performed on a divided m. soleus tendon, the m. gastrocnemius did not produce any plantar flexion. Dividing the m. gastrocnemius tendon did not prevent full plantar flexion. We can conclude then that the plantar flexion is caused by bowing the calf tendons and less so by the proximal displacement of the bellies of the m. gastrocnemius.

A study conducted by Douglas and colleagues came up with other findings. The case study consisted of two patients who were diagnosed with a disruption of the m. gastrocnemius tendon and an intact m. soleus tendon after surgical exploration. They could both actively plantar flex their ankles, but had a positive Simmonds-Thompson test result. This study indicates that the Thompson test is insufficient as a diagnostic of a complete rupture.

Conclusion

While the fundamental principles of the Thompson test have not changed significantly since its introduction, ongoing research continues to validate its effectiveness and explore its underlying mechanisms. The test's simplicity, high sensitivity, and non-invasive nature contribute to its enduring value in clinical practice.