Osseous cyst‐like lesions of the distal interphalangeal joint ‐ how do we assess significance?

The accompanying case report by Fairburn et al. (2024) describes osseous cyst-like lesions (OCLLs) in an atypical location in the palmar processes of the distal phalanx. The authors demonstrate the use of multimodal diagnostic imaging to confirm the diagnosis and clarify the significance of the OCLLs. Two cases demonstrated communication between the OCLLs and the distal interphalangeal joint, which was confirmed by contrast arthrography. In this case series, all three horses with OCLLs in this location were lame, and the OCLLs were judged to be a contributing cause of the lameness in all cases. OCLLs within the distal interphalangeal joint are relatively uncommon, with 7 of 550 horses (1.6%) in one large study of lame horses having OCLLs identified within the distal interphalangeal joint on magnetic resonance imaging (MRI) (Gutierrez-Nibeyro et al., 2020). Similar proportions (1.9%) were reported in a more recent study by Fairburn et al. (2022). OCLLs are most commonly reported on the sagittal midline in the central third of the articular margin of the distal phalanx (Fairburn et al., 2022; Peter et al., 2018). A smaller proportion of OCLLs were reported in the medial and lateral articular surfaces of the joint. OCLLs have also been described at the insertion of the collateral ligaments of the distal interphalangeal joint onto the distal phalanx, both with and without concurrent collateral ligament desmopathy. The unusual location of the OCLLs in the study by Fairburn et al. (2024) raises the question if an atypically located OCLL is more likely to be a significant cause of lameness than one located on sagittal midline? OCLLs may be challenging to detect radiographically and advanced imaging has been shown to detect OCLLs which are not visible by other means (Mair & Sherlock, 2008). A recent study showed that the detection of OCLLs radiographically had a sensitivity of 74.1% and a specificity of 77.9% (Sonnichsen et al., 2023). The atypically located OCLLs described by Fairburn et al. (2024) were most consistently identifiable in PaPrPaDiO views, and may have been overlooked were it not for the use of advanced imaging. This raises the question is advanced imaging necessary to detect OCLLs? OCLLs are rarely reported in non-lame limbs, and a recent study by Peter et al. (2018) showed that in a cohort of Thoroughbreds with OCLLs in the distal phalanx were significantly less likely to race than their maternal siblings and if they raced, their performance was significantly poorer. Sonnichsen et al. (2023) reported that of 104 limbs with OCLLs associated with the distal interphalangeal joint, only 11 were in sound limbs, two of which were horses that were lame on the contralateral limb. This suggests that the presence of an OCLL in the distal interphalangeal joint is likely to be associated with lameness. Given the moderate sensitivity and specificity of radiographic detection, advanced imaging is advocated for accurate characterisation of these lesions. MRI is commonly used to image the feet of horses who respond to diagnostic analgesia of the foot, and its usefulness in diagnosis of OCLLs is well documented (Mair & Sherlock, 2008; Sherlock, 2011). The increasing availability of standing fan beam and cone beam computed tomography (CT) in equine practice has resulted in many more horses undergoing CT imaging of the feet than previously. The appearance of OCLLs within the distal phalanx on CT examination has been described and CT examination has been utilised to characterise fractures of the distal phalanx involving OCLLs (Ball et al., 2020; Del Chicca et al., 2008). Both CT and MRI undoubtedly have a place in diagnosing OCLLs within the hoof capsule, but can they help us to assess their significance? Concurrent pathology within the distal interphalangeal joint and soft tissue structures is commonly present in horses with OCLLs of the distal phalanx, as with case 3 in the accompanying case series. The use of MRI can help us to determine whether significant soft tissue pathology is present, such as deep digital flexor tendinopathy or navicular pathology, which may have an effect on long-term prognosis. MRI is also able to characterise the appearance of the surrounding bone, identifying sclerosis or bone oedema-like signals surrounding the OCLLs, which are indicative of the level of osseous remodelling within the surrounding bone. This may give an indication as to the significance of an OCLL and help to guide the overall prognosis. CT examination is excellent at detecting OCLLs and is useful for detect other articular pathology, such as osteoarthropathy. The use of CT arthrography enabled identification of communication between the OCLLs and distal interphalangeal joint in the accompanying case series and enabled the detection of other articular cartilage pathology. The available cone beam CT systems for imaging the equine foot have limited soft tissue resolution, and therefore soft tissue lesions are unlikely to be identified. Fan beam CT systems have good soft tissue definition, and contrast arteriography can be used as an additional method of detecting soft tissue lesions. Although CT is good at detecting the presence of sclerosis surrounding a lesion, it is not possible to detect bone-oedema-like signal in most currently available CT systems, which limits assessment of the surrounding bone. Sequential examinations with either MRI or CT are useful to monitor the progression of the OCLLs and other accompanying pathology. The author has seen a case of an atypically located OCLL within the distal phalanx, similar to those described by Fairburn et al. (2024). The horse underwent MRI examination approximately 3 weeks after acute onset of lameness, at which time the OCLL was diagnosed and communication with the distal interphalangeal joint was confirmed. Approximately 10 months later, the horse remained lame. Repeat radiological examination did not reveal any significant alteration in the appearance of the lesion. Repeat MRI examination revealed mild enlargement of the OCLL, with better definition of the margins of the lesion and increased sclerosis of the surrounding palmar process of the distal phalanx (Figure 1). No concurrent pathology was identified, and the OCLL was judged to be the primary cause of lameness. The horse was euthanased approximately 6 weeks after the second MRI examination due to ongoing lameness. The repeat MRI examination documented the progression of the lesion and confirmed that this was the likely cause of ongoing lameness. Returning to the initial question—how do we assess the significance of OCLLs in the distal phalanx? OCLLs are likely to be a significant cause of lameness when present, both in a typical and atypical location, and accurate diagnosis by radiography alone may not be possible. Concurrent pathology is frequently present and is more fully assessed by MRI than radiographs. The use of MRI and/or CT examination and arthrography (radiographic or CT) provides valuable information about location and dimensions of OCLLs and the presence of articular communication as well as assessment of the surrounding bone and other structures of the foot. This provides valuable information for prognosis and treatment planning. A multimodal approach to the diagnosis of OCLLs is advised in order to fully appreciate the extent of the lesion, the likelihood of concurrent pathology and to accurately assess prognosis for future soundness. L. J. Meehan-Howard: Writing – original draft; writing – review and editing. There are no funders to report for this submission. No conflicts of interest have been declared.