Przejdź do treściComparing the painful side with the asymptomatic side can help distinguish an individual anatomical feature from a true abnormality. It is not, however, an automatic deciding test. The opposite limb may contain asymptomatic changes, and some disorders are bilateral.
Each patient provides their own reference point
Reference ranges for tendon thickness or nerve size are useful, but they do not capture the full range of biological variation. Comparing both sides can show whether a difference is pronounced in that particular patient. This is especially useful when the finding is subtle or lies close to the boundary of what is considered normal.
What can an orthopaedic surgeon compare during ultrasound?
- tendon thickness and internal structure,
- nerve cross-sectional area and echogenicity,
- the amount of fluid in a bursa or tendon sheath,
- how a structure behaves during movement,
- vascularity on Doppler imaging,
- bony contours and anatomical relationships.
Side-to-side comparison is most useful when the images are obtained in the same plane, with a similar limb position and identical machine settings. Otherwise, an apparent asymmetry may reflect technique rather than disease.
Ultrasound is not only a static image
One of the advantages of musculoskeletal ultrasound is the ability to observe tissues in real time. A tendon may snap only during a particular movement, a nerve may change position as a joint bends, and tissue impingement may appear only in a specific position. Comparing both sides during the same manoeuvre can therefore be more informative than comparing two isolated still images.
Why is the other side not always a “healthy control”?
Imaging abnormalities can occur without pain. This applies, among other things, to degenerative changes in tendons, entheses and menisci. Conversely, a disorder may affect both sides even when symptoms are present only on one side. A symmetrical appearance therefore does not exclude disease, and asymmetry by itself does not prove that the source of pain has been identified.
The image must be matched to the exact site of symptoms
The greatest value comes from combining three elements: the history, physical examination and ultrasound findings. If an abnormality is located where the patient hurts, changes during the movement that provokes symptoms and fits the clinical mechanism, it becomes more likely to be relevant. If it is incidental and does not correspond to the symptoms, it should not determine treatment on its own.
This approach was also important in a study of 430 patients evaluating meniscal abnormalities with ultrasound, co-authored by Tomasz Poboży. Ultrasound was treated as one component of the clinical assessment, with attention to the type of symptoms and the limitations of the method.
When can side-to-side comparison be misleading?
- after a previous injury or operation on the currently asymptomatic side,
- in bilateral or systemic disorders,
- when the limb or transducer position is not equivalent on both sides,
- when the difference lies within normal biological variation,
- when the visible abnormality does not match the location of symptoms.
Practical takeaway
The opposite side is a valuable reference, but it does not replace knowledge of anatomy, diagnostic criteria or examination of the patient. Good ultrasound practice is not about finding any asymmetry. The question is whether the observed difference actually explains the symptoms and whether it should change further management.
More about: MSK Ultrasound and Ultrasound-Guided Procedures
References
- Poboży T, Konarski W, Janowski K, Michalak K, Poboży K, Domańska-Poboża J. Ultrasonographic Assessment of Meniscus Damage in the Context of Clinical Manifestations. Medicina. 2025;61(8):1339. DOI: 10.3390/medicina61081339.
- European Society of Musculoskeletal Radiology. Musculoskeletal ultrasound technical guidelines. Current technical recommendations and regional scanning protocols.
This material is educational and does not replace an individual medical consultation.
Content prepared by: Tomasz Poboży - orthopaedic surgeon.