Iliopsoas strain demographics, concurrent injuries, and grade determined by musculoskeletal ultrasound in 72 agility dogs

Abstract

The objective of this study was to describe patient demographics associated with iliopsoas strains, frequency of common concurrent injuries, and associated strain grades based on musculoskeletal ultrasound.

The medical records of 72 client-owned agility dogs that had an iliopsoas musculoskeletal ultrasound (MSK-US) between 2009 and 2015 were retrospectively reviewed. Analyses included patient signalment, physical examination, and diagnostic findings.

Twenty-four breeds of canine athletes from 1.5 to 10 y old (median: 5 y, SD: 2.2 y) were included in the study. Of the 72 records reviewed, border collies were the most common breed (27.8%, 20/72) reported. Isolated iliopsoas strains occurred in 26.4% (19/72) of cases. Concurrent pathology was noted in 73.6% (53/72) of cases. Cranial cruciate ligament (CCL) instability was the most common concurrent pathology, representing 27.8% (20/72) of all cases, with hip (8.3%, 6/72), lumbosacral (23.6%, 17/72), other non-CCL hind limb (6.9%, 5/72), and forelimb (6.9%, 5/72) pathologies making up the remainder of cases with concurrent pathology. In patients with a concurrent hind limb injury, 96.7% (30/31) of dogs had their most severe iliopsoas strain grade on the same limb.

MSK-US revealed Grade I strains in 54.2%, Grade II strains in 22.2%, Grade III strains in 5.2%, and chronic changes in 18.1% of cases. There were no statistically significant associations between iliopsoas strain grade and age, body weight, sex, breed, concurrent pathology, anatomic location of concurrent pathology, or sidedness of concurrent pathology.

Iliopsoas strains are one of the most common agility dog injuries; however, patient demographics, prevalence of concurrent injury and correlation with MSK-US findings have not been previously reported. To the authors’ knowledge, this is the first retrospective analysis reporting iliopsoas strain demographics, concurrent injury frequency and correlation with MSK-US evaluation in agility dogs. Although 26.4% of iliopsoas strains occurred as isolated injuries, 73.6% had concurrent injuries, with CCL instability present most commonly, occurring in 27.8% of cases.

Dogs should be thoroughly evaluated for concurrent injuries when presenting with an iliopsoas strain.

Introduction

Iliopsoas injury is a common cause of hind limb lameness in dogs and has been investigated at an increased frequency in recent years (1–4). In a study by Nielson and Pluhar (3), the iliopsoas muscle was the most common hind limb muscle strain, primarily affecting 32% of cases. In addition, in that study, 95% of all hind limb muscle strains were localized to the hip adductor muscle group, comprised of iliopsoas and/or pectineus muscles, and dogs were markedly uncomfortable on simultaneous hind limb extension and internal rotation (but with pain unable to be isolated to either the iliopsoas or pectineus muscles). In a more recent study by Markley et al (5) that surveyed 4197 agility dog owners, iliopsoas strain was the second most often reported condition, occurring in 19.4% of the 1958 canine athletes included.

The iliopsoas muscle is comprised of the iliacus and psoas major muscles. The psoas major originates from the transverse processes of lumbar vertebrae 2 and 3, with additional connections to lumbar vertebrae 3 through 7 via an aponeurosis with the quadratus lumborum muscle. The iliacus muscle arises from the ventral ilium. The 2 muscles course caudoventrally with a conjoined insertion on the lesser trochanter of the femur. The major function of the iliopsoas is hip flexion. In addition, the iliopsoas aids in hip joint and vertebral column stabilization, flexion of the vertebral column when the femur is in a fixed position and produces adduction and external rotation of the femur (6,7).

The frequency of muscle injury in dogs is presumed to be underdiagnosed due to the often self-limiting nature of the injury and the patient not being presented to a veterinarian (2,3,6,8). Likewise, an iliopsoas strain may also act as a “red herring,” and be implicated as the suspected primary cause of discomfort, when comorbidities may be present, such as a cranial cruciate ligament rupture, underlying hip dysplasia, or lumbosacral pathology. Orthopedic or neurologic causes of lameness, such as cranial cruciate ligament injury (CCL), hip dysplasia (HD), lumbosacral (LS) disease, or spinal myelopathy, may confound the diagnosis and occur concurrently with an iliopsoas strain (1,3,9,10).

Acute iliopsoas strains occur when external forces applied to the muscle overload its contractile force during eccentric contraction (3,4,8,11,12). Strains commonly occur at the muscle-tendon junction as this is the weakest portion of the myotendinous unit. This results in muscle fiber disruption and loss of continuity, disruption in vascular supply, and subsequent interstitial hemorrhage and inflammation (3,13). Patients with acute iliopsoas injury may have an identifiable preceding traumatic event such as jumping, slipping into a splay-legged position in the hind limbs, or strenuous hiking, and are often associated with athletic events such as agility (1). Chronically affected patients typically have a history of recurrent or progressive hind limb lameness (2). Furthermore, a patient with iliopsoas injury may ambulate with a shortened stride length and decreased hip extension (2). Agility dogs may exhibit decreased hind limb extension when jumping, decreased drive in their hind limbs when weaving, and decreased weight bearing in the affected limb. Pain and/or spasm may be appreciated on palpation of the lumbar muscles, ventromedial to the ilium at the lesser trochanter, and on transrectal palpation of the pubic rim in small dogs. They also may be resistant to hip extension with simultaneous internal rotation (iliopsoas muscle maneuver test), or hip abduction (1,3,4).

The structures evaluated during musculoskeletal ultrasound (MSK-US) of the iliopsoas include the region of insertion on the lesser trochanter and its associated bursa, the muscle belly, and muscle origins. Examination allows for grading of the strain and subsequent categorization, according to a 3-tiered grading system based on structural abnormalities and strain severity (4,14,15). Muscle and tendon injury are graded according to the same scheme.

Ultrasonographically, acute strains appear as hypoechoic lesions within the belly of the muscle, or within the tendon, and there is evidence of fiber disruption. Chronic strains appear hyperechoic, indicative of fibrosis or scar tissue, presumably a consequence of a historic acute strain, acute on chronic injury or repetitive microtrauma (4). Grade I strains have focal edema or hemorrhage with < 5% muscle or tendon involvement. Grade II strains have increased muscle and tendon involvement, with mild disruption of muscle fibers, and increased presence of edema and/or hemorrhage. In Grade III strains, there is marked fascial tearing, muscle and tendon fiber rupture, edema and/or hemorrhage. In instances of chronic injury, similar degrees of fiber disruption and muscle involvement are present as seen in Grade II strains, but with hyperechoic fiber changes within the muscle/tendon and/or fibrous change/debris within the iliopsoas bursa (4,12,16). Dogs with injuries containing properties of 2 grades are given an intermediate grade (i.e., Grade I–II or II–III) (4).

Iliopsoas strains are a common injury in agility dogs, yet there is a paucity of literature on the topic. The goals of this study were to describe the demographics and frequency of concurrent injuries with iliopsoas strains in agility dogs. We hypothesized that dogs with iliopsoas strains have concurrent musculoskeletal pathology in > 50% of cases.

Materials and methods

Medical records of 72 agility dogs that were presented to Veterinary Orthopedic and Sports Medicine Group between 01/2009 and 7/2015 with pain on iliopsoas palpation, iliopsoas stretch (i.e., hip extension with internal rotation) and a confirmed iliopsoas strain based on MSK-US were reviewed. The present study is a follow-up to a study performed by Cullen et al (4) that used the same patient population and described the ultrasound findings in agility dogs with iliopsoas strains and proposed a 3-tier grading scheme for iliopsoas strain characterization based on MSK-US findings. The owner of 1 of the initial 73 patients included in the initial study had requested to be excluded in subsequent research studies; the remaining 72 patients were included in the present report. Items reviewed included patient demographics, concurrent musculoskeletal conditions noted on physical examination, as well as diagnostics pursued based on examination findings. Concurrent injuries were categorized based on physical examination and diagnostic imaging findings. All patients were sedated for MSK-US evaluation with a combination of butorphanol 0.2 mg/kg and dexmedetomidine 3 μg/kg IV or IM. All ultrasound examinations were performed by a single observer (Debra Canapp ‘DC’). MSK-US findings were used to grade iliopsoas strains into 4 categories (I, II, III, and chronic) as previously described, and are summarized in Table I (10). Iliopsoas strains with characteristics overlapping 2 grade categories (i.e., Grade I–II strain) were rounded to the nearest grade (i.e., a Grade II–III rounded to a Grade III lesion) based on discussion with the ultrasonographer (DC) to facilitate analysis. Patients were allocated into 6 categories: no concurrent injury (isolated iliopsoas strain), concurrent CCL instability, hip pathology, LS disease, forelimb pathology, and other non-CCL hind limb injury. Cranial cruciate ligament pathology was diagnosed on physical examination based on palpable tibial thrust or cranial drawer, or via arthroscopy. Hip pathology was diagnosed based on laxity appreciated while performing an ortolani maneuver, or if there was a radiographic diagnosis of hip dysplasia with concurrent pain on hip extension. Lumbosacral pathology was suspected when there was significant discomfort appreciated on palpation of the LS junction on physical examination. Magnetic resonance imaging reports from patients that had undegone LS imaging were reviewed to determine if LS discomfort was a result of spinal cord compression, nerve compression, or referred pain from the iliopsoas origin along the lumbar vertebrae. All patients received a static position MRI.

Table I.

Musculotendinous grading scheme for dogs.

Grade	Description
I	“Mild strain,” < 5% muscle involvement, focal edema/ hemorrhage
II	“Moderate strain,” > 5% muscle involvement, mild fiber tearing, increased edema/hemorrhage
III	“Severe strain,” significant fascial tearing, marked to complete muscle fiber disruption, marked edema/ hemorrhage
Chronic	Hyperechoic fiber pattern, hyperechoic changes and/or debris within bursa, > 5% muscle involvement, mild fiber tearing

Statistical analyses were used to identify potential relationships between selected parameters and iliopsoas injury grade. Categorical data including sex, breed, concurrent injury diagnosis, anatomic location of concurrent injury (i.e., hind limb, forelimb, or LS), and sidedness of concurrent injury diagnosis, compared to the iliopsoas injury, were evaluated using a Pearson’s Chi square. Continuous data (age and body weight) were assessed using Pearson’s correlation. Statistical significance was set at P ≤ 0.05 for all analyses. Analyses were performed using NCSS 2022, v22.0.3 (NCSS 2022 Statistical Software (2022). NCSS, LLC. Kaysville, Utah, USA, ncss.com/software/ncss).

Results

All canine athletes were from 1.5 to 10 y old, with a mean age of 5.0 y (SD = 2.2 y). Twenty-four dog breeds were represented. The border collie was the most common purebred dog (n = 20), followed by the Australian shepherd (n = 8), Shetland sheepdog (n = 5), and golden retriever, Labrador retriever, and the Welsh corgi (n = 4 each) with various other purebred dogs reported in fewer numbers (< 4 individuals each). The average dog weight was 20.2 kg, with 56 patients weighing from 10 to 30 kg (78%). Spayed females were most common in this group representing 32 of 72 (44.4%) participants. Neutered males accounted for 18 (25%) dogs, and there were 8 intact females (11%) and 14 intact males (19.4%).

Of the 72 patients examined, a concurrent, non-iliopsoas injury was diagnosed in 53 dogs. Distributions of the suspected concurrent musculoskeletal conditions based on physical examination are summarized in Table II. In 19 (26.4%) dogs, the only source of discomfort was pain on palpation of the iliopsoas insertion. In all 19 cases, the iliopsoas strain was confirmed to be the only musculoskeletal pathology, and therefore determined to be a primary iliopsoas strain. Cranial cruciate ligament instability was present in 20 dogs (27.8% of cases). Lumbosacral discomfort was noted on palpation in 22 patients but was considered the primary source of pain based on reactivity during lumbosacral palpation in 17 patients (23.6% of all cases). Magnetic resonance imaging of the LS spine was performed in 14 of the 17 patients in which the LS region was localized as the primary source of pain. Of the 14 patients, MRI revealed LS pathology in 5 patients and was normal in 6 patients. Reports were unavailable in 3 patients at the time of the present study. The distribution of confirmed injuries (n = 66), considering MRI findings, is shown in Figure 1. In this subset of data, dogs with a normal LS scan were considered to have a primary iliopsoas strain, except for 1 dog with very mild concurrent CCL pathology (but no appreciated instability), with significant lumbosacral pain on initial physical examination. Dogs with significant LS discomfort without an MRI performed, or dogs with unavailable MRI reports, were not included in this data subset. Radiographically diagnosed hip pathology and pain and/or instability was noted in 6 dogs (8.3% cases). An injury to the forelimb was noted in 5 dogs (6.9% of cases), and a non-CCL injury to the hind limb was diagnosed in 5 dogs (6.9% of cases). Forelimb injuries consisted of medial shoulder instability (n = 2), supraspinatus and biceps tendinopathy (n = 2), and fragmented medial coronoid process (n = 1). Non-CCL hind limb injuries included a sartorius strain (n = 1), and medial patellar luxation with marked stifle osteoarthritis (n = 4).

Table II.

Anatomic distribution of injury based on physical examination (n = 72).

Anatomic location
Iliopsoas insertion	19 (26.4%)
Iliopsoas plus:
CCL	20 (27.8%)
Hip	6 (8.3%)
Lumbosacral	17 (23.6%)
Forelimb	5 (6.9%)
Non-CCL hind limb	5 (6.9%)

Figure 1.

Distribution of confirmed primary injuries (n = 66). Note, 6 cases were removed due to no MRI report (n = 3) or not pursuing MRI (n = 3) despite LS pain on physical examination.

In all patients, MSK-US of the iliopsoas was performed bilaterally. Distribution of all iliopsoas strain grades is illustrated in Figure 2. MSK-US examination of the iliopsoas revealed Grade I strains most often in 39 cases (54.2%). Grade II strains were present in 16 cases (22.2%), Grade III strains in 4 cases (5.6%), and chronic changes in 13 cases (18.1%). Bilateral ultrasonographic iliopsoas changes were noted in 52/72 (72.2%) of dogs. Although frequently occurring, contralateral ultrasonographic findings in unilaterally lame dogs were thought to be less significant and were considered a compensatory, non-clinical finding. In dogs with concurrent pathology, the most significant iliopsoas injury, determined by both reactivity on physical examination and MSK-US findings, occurred on the same hind limb as the concurrent pathology most seen in 30 cases (41.7%). The concurrent pathology occurred in the contralateral hind limb in 1 (1.4%) case, the ipsilateral forelimb in 3 (4.2%) cases, and the contralateral forelimb in 2 cases (2.8%), as illustrated in Figure 3. In 17 cases (23.6%), pain was localized to the LS region on spinal palpation, and therefore was not a lateralizing physical examination finding. Of the 31 patients in which the primary injury was localized to the hind limb, 30 (96.8%) of patients had their most significant iliopsoas strain on the same limb of their concurrent injury. Of the 14 patients that had an MRI of the LS spine, 11 reports were available for review. The primary source of LS pain was attributed to an isolated iliopsoas strain in 6/11 cases (54.5%) as no nerve or spinal cord compression was noted. In the 5 (45.5%) remaining MRI cases, iliopsoas pain was deemed secondary to underlying intervertebral disc disease or nerve impingement.

Figure 2.

Distribution of all iliopsoas pathology noted on musculoskeletal ultrasound (n = 72).

Figure 3.

Sidedness of concurrent injury relative to the primary iliopsoas strain (n = 72).

Grade I iliopsoas strains were most common in patients with isolated iliopsoas strains, and concurrent CCL, hip, LS pathology, or other non-CCL hind limb injuries. In the studied sample population, Grade I strains were equally as frequent in those with concurrent forelimb pathology as Grade II strains. Grade II strains were diagnosed in 16 dogs, with 7/16 (43.8%) occurring in dogs with isolated iliopsoas strains. Grade III strains were the least frequently confirmed strain in all categories. Chronic changes to the iliopsoas occurred most frequently in dogs with concurrent CCL instability 4/13 (30.8%). These findings are summarized in Table III and illustrated in Figure 4.

Table III.

Distribution of iliopsoas strain grade based on primary injury on physical examination (n = 72 dogs).

	Isolated iliopsoas	CCL	Hip	Lumbosacral	Forelimb	Other non-CCL hind limb
I	8	13	4	9	2	3
II	7	2	0	4	2	1
III	1	1	1	1	0	0
Chronic	3	4	1	3	1	1

Figure 4.

Distribution of iliopsoas strain grade based on isolated or concurrent injury on physical examination (n = 72).

There were no significant differences amongst sex, breed, or concurrent injury type, anatomic location of concurrent injury, or sidedness of concurrent injury between the 4 iliopsoas injury categories in this dataset. Likewise, there were no significant correlations between age or body weight, and any of the 4 iliopsoas injury categories, or between sidedness of iliopsoas injury and concurrent injuries in cases in which those were present.

Discussion

Results of this study confirmed that most agility dogs with iliopsoas strains had concurrent injuries. Of the concurrent injuries noted on physical examination, CCL injury was the most common, closely followed by LS disease. Pathology associated with the hip, forelimbs and other hind limb and non-CCL injuries were also reported, but less frequently. Notably, LS pain was reported in 17 cases; however, in only 5 cases of the 14 that received an MRI, was pathology of the spine or nerves diagnosed. This is likely underdiagnosed in the studied population as an LS MRI was not performed in all patients and is a limitation of this study. In addition, all patients received a static MRI. Perhaps subclinical lumbosacral pathology was present in some dogs whose lumbosacral region was deemed normal on MRI, whereas pathology may have been diagnosed had a dynamic MRI been performed (17). Further research is warranted to evaluate the frequency of these 2 concurrent pathologies due to the overlap in physical examination findings and relatively low number of cases for these injuries.

Consistent with previous studies and breed popularity in agility, the border collie was the most common purebred dog in this study (5,18,19). The 2021 report by Markley et al (5) highlighted the iliopsoas muscle as a common injury amongst agility dogs. In that study, the iliopsoas was the second most frequently reported injury, behind shoulder injury, accounting for 19.4% of injuries sustained. In a report by Nielsen et al (3), the iliopsoas was the most common pelvic limb muscle strain location. Iliopsoas strains had been reported to occur concurrently with other concurrent orthopedic and neurologic diseases (1,3), and as reported in a study by Ragetly et al (8). Despite the common nature of this disease in agility dogs, there is a dearth of studies investigating iliopsoas strains. To the authors’ knowledge, this is the first report investigating the frequency of iliopsoas strains as either an isolated or concurrent injury in canine athletes, as well as the distribution of iliopsoas strain grades relative to concurrent injury, or lack thereof.

Treatment and recovery of iliopsoas strains was outside the scope of this report due to the frequency of concurrent injury and frequent loss of follow-up after diagnosis of a concurrent injury. Treatment and management of iliopsoas strains is variable, based on both strain grade and clinician preference, and the timeline to recovery is often confounded by concurrent injuries. Accordingly, further research is warranted regarding the ideal treatment for this condition.

A limitation of the present study was the retrospective nature and reliance on accuracy and completeness of medical records. Data included in this study was between 7 and 13 y old at the time of this report; therefore, diagnostic techniques and interpretation may have been altered based on continued clinical experience, advancements in research, and diagnostic technologies. Additional limitations included lack of blinding, randomization, control group, and MRI verification of iliopsoas strain diagnosis. In addition, there is risk of bias as data were collected by the principal investigator, who was not the same investigator who obtained all ultrasound images. For the purposes of this study, injury grades were divided into 4 categories (I, II, III, and chronic). As previously reported, intermediate grades (i.e., I–II) for iliopsoas strains have historically been assigned to facilitate analysis with the selected population (4). To facilitate statistical analysis, grades were rounded up to the nearest grade (i.e., a Grade II–III rounded to a Grade III lesion) for all injuries originally classified as an intermediate grade, based on discussion with the ultrasonographer (DC). Therefore, the resulting grades in this study were slightly more severe from what truly occurred in the studied population. Classification of strain chronicity was based on ultrasonographic appearance, correlated with clinical experience and previous retrospective studies, including timeline of injury, muscle/tendon fiber pattern, and presence of edema on ultrasound. No histologic testing was performed to corroborate MSK-US findings. All these factors could have contributed to the lack of statistically significant associations between iliopsoas injury grade and the parameters examined in this dataset, although it is also important to note that some of the comparisons made were underpowered. Additional work in larger groups of dogs is needed to confidently determine if any of these parameters are significantly associated with grade of iliopsoas strain in the dog, and whether any of these relationships are clinically relevant.

Another significant limitation to this study was case selection. Despite no available data on the overall percentages of iliopsoas strain grades in companion animals, there may be an increased frequency of high-grade strains in canine athletes due to frequent microtrauma related to their high activity. All cases were obtained from a single facility; therefore, most cases were from a relatively similar geographic region within the United States. As described by Markley et al (5), iliopsoas injury is significantly more common in the US and Canada than in dogs of the UK, Ireland, and continental Europe. This may have further implications in distribution of iliopsoas strain severity and correlation with concurrent injuries. In addition, the reported concurrent injuries are not isolated to only those participating in agility. Data are lacking in the reported injury rates within the overall canine population. The present study did not compare the frequency of concurrent injuries as related to that of the general population.

Findings of this study highlighted the complicated nature of diagnosing iliopsoas strains on physical examination only, and the importance of utilizing advanced imaging techniques such as MSK-US to confirm and grade an iliopsoas strain. In addition, the high frequency of concurrent injuries indicated the importance of screening for other concurrent pathology when iliopsoas pain is noted on physical examination.