Intermediate patellar ligament desmopathy often occurs in conjunction with other stifle abnormalities

The equine patellar ligaments and the infrapatellar fat pad - a microanatomical study

BACKGROUND

Interpretation of patellar ligament (PL) ultrasonography may be difficult, as hypoechoic or heterogenous echogenicity are common findings. Verifying suspected disease of equine PLs by histopathology is also problematic as descriptions of normal PL vascularity and histology are scarce. The current study describes the PL and infrapatellar fat pad (IFP) vascular pattern from computed tomography scans of barium perfused normal equine specimens (n = 8; age 10 days to 18 years), as well as routine histology to serve as a reference for future investigations into PL pathology and IFP disease.

RESULTS

The PLs received a bipolar blood supply. Vascular architecture consisted of numerous distinct longitudinal vessels with several horizontal connections, which branched into extensive latticeworks of smaller vessels throughout the ligaments. Several vascular connections between the PLs and the IFP were identified. One distinct longitudinal vessel was seen entering each of the IFP lobes at the distocranial aspect, branching extensively into lobar vascular networks which anastomosed by several horizontal branches at the mid portion of the IFP where the two lobes merge. Histologically, there were large variations in PL interfascicular endotenon thickness, vascularity and fatty infiltration; these parameters increased with age for the intermediate and medial PL. Areas of metaplastic tenocytes / chondroid metaplasia were identified in all investigated adult medial PLs; in 2/7 in the intermediate PL and in 4/7 in the lateral PL. The adult IFP consisted of white unilocular adipose tissue, organized in lobules separated by thin connective tissue septa increasing in thickness towards the periphery and the distocentral aspect.

CONCLUSIONS

The equine PLs and IFP are highly vascularized structures with ample vascular connections suggestive of crosstalk. This, together with the large variation in PL endotenon thickness, vascularity and fatty infiltration, should be taken into consideration when assessing potential PL histopathology as these changes increase with age and are found in horses without clinical signs of stifle disease. Metaplastic tenocytes / chondroid metaplasia should be considered a normal finding throughout the medial PL and is not age dependent. The role of the equine IFP in stifle disease has yet to be elucidated.

Ultrasonographic comparison of the intermediate patellar ligament in warmbloods versus quarter horses

Striations are present on ultrasonography of the intermediate (middle) patellar ligament in the transverse plane, which can be confused with tears. Comparison to the contralateral limb is often performed to help differentiate anatomic variation from pathologic change. The purposes of this prospective, observational study were to describe the striation patterns in Warmbloods and Quarter Horses, determine if these patterns are bilaterally symmetrical, and compare striation characteristics between Warmbloods and Quarter Horses. The intermediate patellar ligaments of six Warmblood horses and six Quarter horses, free from clinical signs of hindlimb lameness and in full work, were examined ultrasonographically. Striation pattern, striation number, and the transverse-sectional areas for intermediate patellar ligaments were compared between Warmbloods and Quarter horses. Striation patterns were also compared for the left and right limbs of each horse. A significant difference between breeds was identified at the mid-portion of the intermediate patellar ligament when both left and right ligaments were included (P = 0.02) and when comparing the right intermediate patellar ligament (P = 0.02). There were no other significant breed differences, and a parallel pattern was the most common pattern type. Two-thirds of the horses in this study had bilaterally symmetrical patterns within the distal aspect of their intermediate patellar ligaments, while one-third of the population were asymmetric. These results show that comparison of the contralateral limb during ultrasonography to determine if changes within the distal aspect of the intermediate patellar ligament are normal striations versus tears may lead to misdiagnosis in one-third of horses, regardless of breed. Other signs of pathology on ultrasonography should be used instead.

Patellar ligament desmopathy in the horse – a review and comparison to human patellar tendinopathy (‘Jumper’s knee’)

Patellar ligament desmopathy in horses is regarded as an uncommon condition with unclear aetiology. Of the three patellar ligaments in the horse, the intermediate is the one most often diagnosed with desmopathy in horses presenting with chronic lameness. This structure corresponds to the patellar tendon in humans. As diagnostic imaging modalities continuously improve, changes in echogenicity of the patellar ligaments are identified ultrasonographically with increasing frequency. However, disruption of the normal fibre pattern may be present also in patellar ligaments in horses that show no signs of lameness. Similarly, there is a poor correlation between pain and diagnostic imaging findings in human patellar tendinopathy. Consequently, there appears to be a knowledge gap pertaining to normal ultrasonographic variation and diagnostic criteria for disease of the patellar ligaments in horses. Furthermore, local anaesthetic techniques to verify the diagnosis are poorly described, and due to the low number of treated cases, no specific treatment modality can be recommended on a scientific basis. The aim of this paper is to review the current knowledge regarding the pathogenesis, diagnosis and management of patellar ligament desmopathy in horses, compare this condition with patellar tendinopathy in humans, and identify areas for further research to increase the diagnostic accuracy in horses. We conclude that there is a profound need for better descriptions of ultrasonographic variation and pathological changes of the equine patellar ligaments. Identification of areas of maximal ligament strain and descriptions of early histopathological changes could render more information on the possible aetiology, preventive measurements and treatment options of desmopathy. Description of regional innervation could aid in development of methods for diagnostic anaesthesia to verify pain originating from the ligaments.