Evaluation of a cranial arthroscopic approach to the stifle joint for the treatment of femorotibial joint disease in horses: 23 cases (1998-1999)

OBJECTIVE: To describe a single-portal cranial arthroscopic approach to the stifle joint in horses and to determine the clinical outcome in horses with femorotibial joint disease in which this approach was used. DESIGN: Retrospective study. ANIMALS: 23 adult horses. PROCEDURE: Medical records were reviewed to obtain information on clinical outcome in horses in which the single-portal cranial arthroscopic approach was used. RESULTS: Twenty-nine stifle joints of 23 horses were examined arthroscopically, using the described approach. Subchondral bone cysts were treated in 19 medial femoral condyles of 12 horses. Unilateral cruciate ligament desmitis (4 horses), meniscal tearing (3), or both (2) were identified in 9 horses. Evidence of degenerative joint disease without cystic lesions or soft tissue trauma was found in 2 horses. Information on clinical outcome was obtained for 21 of 23 horses. A successful outcome was obtained in 15 of 21 horses and was defined as return to sound performance at a degree equal to or better than that prior to injury and lameness. Eight of 12 horses treated for medial femoral condylar cysts had a successful outcome. Four show horses treated for cruciate ligament lesions alone successfully returned to showing activity. None of the 3 horses with meniscal tearing were able to perform successfully. CONCLUSIONS AND CLINICAL RELEVANCE: The femorotibial joint was evaluated through a single-portal cranial arthroscopic approach, using the femoropatellar joint as the point of access. This approach was easy to perform, allowed controlled access to the femorotibial joint, avoided accidental damage to articular structures, and required fewer access portals.     

Long-Term Outcome in 44 Horses with Stifle Lameness After Arthroscopic Exploration and Debridement

Objective— To (1) examine the outcome in horses with osteoarthritis or intra‐articular soft tissue injuries of the stifle after arthroscopic exploration and debridement and (2) to determine any imaging or surgical findings that may influence prognosis. Design— Case series. Animals— Horses (n=44) with lameness referable to the stifle, diagnosed with osteoarthritis, meniscal tears, or other intra‐articular soft tissue injuries based on arthroscopic examination. Methods— Medical records of horses with stifle lameness that had arthroscopic exploration were reviewed. Horses with osteochondrosis lesions, intra‐articular fractures, or osseous cyst‐like lesions were excluded. Pertinent case information was analyzed and short‐ and long‐term outcome was assessed. Results— There was no association between radiographic score and surgery score. Diagnostic ultrasound had a sensitivity of 79% and a specificity of 56% for identifying meniscal injuries. Follow‐up information was available for 35 horses; 23 horses (60%) improved after surgery, 16 (46%) became sound, and 13 (37%) returned to their previous level of function. A negative association was observed between age and degree of preoperative lameness and outcome. More severe changes observed on preoperative radiographs were also negatively associated with prognosis. No horses with grade 3 meniscal tears improved postoperatively and increasing meniscal pathology was negatively associated with return to previous function. A weak association between surgery grade and outcome was also observed. Degree of chondral damage, location of primary pathology, and microfracture techniques had no effect on outcome. Conclusions— Advanced horse age, severe lameness and preoperative radiographic changes, and presence of large meniscal tears are associated with a negative postoperative outcome for horses with stifle lameness. Appearance of the articular surface at surgery appears to be an inconsistent prognostic indicator. Clinical Relevance— Some horses with extensive cartilage damage may return to athletic function after arthroscopic debridement and lavage. A more pessimistic prognosis may be given to older horses, those with more severe preoperative lameness, and those with severe radiographic changes or large meniscal tears.     

Meniscal tears in horses: an evaluation of clinical signs and arthroscopic treatment of 80 cases

REASONS FOR PERFORMING STUDY: There is little published information available describing clinical signs, arthroscopic findings and prognosis of meniscal injuries in horses. OBJECTIVES: To evaluate the effect on the outcome not only of the arthroscopic findings and treatment, but also of the clinical and radiographic signs in these horses. METHODS: The following were recorded for each case: the meniscal injury, graded according to severity; clinical and radiographic findings prior to surgery; any concurrent injury in the joint seen at arthroscopy. The effect of these factors and the grade of injury on the outcome were analysed using Fisher's exact test or Chi-square analysis. Only horses whose meniscal injury was judged to be the primary cause of lameness were included in the series. RESULTS: A series of 80 meniscal injuries were diagnosed and treated arthroscopically by the authors at the Liphook Equine Hospital and 47% of horses returned to full use. Statistically, poor prognosis was associated with increasing severity of the meniscal injury, the presence of concurrent articular cartilage lesions and radiographic abnormalities in the joint. Arthroscopic treatment of many lesions was limited by the inaccessibility of parts of the femorotibial joint. POTENTIAL RELEVANCE: Further work is required to improve and evaluate arthroscopic techniques for the treatment of these injuries.     

Diagnostic findings and prognosis following arthroscopic treatment of subtle osteochondral lesions in the shoulder joint of horses: 15 cases (1996-1999)

OBJECTIVE: To determine clinical, scintigraphic, radiographic, and arthroscopic findings and results of treatment in horses with lameness attributable to subtle osteochondral lesions of the shoulder joint. DESIGN: Retrospective study. ANIMALS: 15 horses. PROCEDURE: Medical records were reviewed, and results of physical examination, scintigraphy, radiography, arthroscopy, and treatment were recorded. RESULTS: Severity of lameness ranged from grade 1 to 4. Response to shoulder flexion or extension was variable. Twelve horses had a narrow upright foot. Intra-articular anesthesia of the shoulder joint localized the cause of the lameness to the shoulder joint in 9 of 10 horses. Scintigraphic abnormalities were detected in 4 of 6 horses. Radiographic lesions were subtle and included glenoid sclerosis, focal glenoid lysis, small glenoid cysts, and alterations in the humeral head contour. Arthroscopic evaluation confirmed clefts in the glenoid cartilage, glenoid cysts, a humeral head cyst, fibrillation of the humeral head cartilage, cartilage fragmentation, or a nondisplaced fracture of the humeral head. After treatment, 12 horses returned to their previous level of performance, 1 was sound for light riding, 1 remained lame, and 1 was euthanatized because of chronic lameness. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that a combination of physical examination, scintigraphy, and radiography is necessary to diagnose subtle osteochondral lesions of the shoulder joint in horses. Arthroscopy can be used to confirm the diagnosis and treat cartilage and subchondral bone lesions. Young and middle-aged horses with subtle osteochondral lesions of the shoulder joints have a good prognosis for return to performance following arthroscopic treatment.     

Arthroscopic removal of an osteochondral fragment from the caudal pouch of the lateral femorotibial joint in a colt.

An osteochondral fragment was removed from the caudal pouch of the lateral femorotibial joint in a 2-year-old Trakehner colt by use of arthroscopic surgery and a lateral approach. The approach to this aspect of the femorotibial joints was developed in another horse. The fragment was not attached and resembled an osteochondritis dissecans lesion. The intermittent lameness associated with the fragment resolved after surgical removal. A positive response to diagnostic anesthesia of the femorotibial joint in the absence of a confirmed diagnosis (following radiographic and arthroscopic evaluations of the other aspects of the stifle joint) is an indication for arthroscopic exploration of the caudal pouches of the femorotibial joints of stifles in horses.     

Concurrent or sequential development of medial meniscal and subchondral cystic lesions within the medial femorotibial joint in horses (1996–2006)

Reasons for performing study: Medial meniscal injuries and subchondral cystic lesions (SCL) are known to occur independently within the medial femorotibial (MFT) joint in horses. However, there are no reports of a potential clinical relationship between these 2 types of lesions. Objectives: To: 1) document the concurrent presence or sequential development of medial meniscal and SCL of the medial femoral condyle within the MFT joint; and 2) determine the prognosis with both types of lesions. Methods: Retrospective case series of horses with both a medial meniscal and SCL of the medial femoral condyle identified concurrently or sequentially by radiography, arthroscopy or post mortem examination. Case records and radiographs were reviewed, and a telephone survey of referring veterinarians, owners and trainers was conducted. Results: Twenty‐one horses (9.1% of all horses undergoing MFT joint arthroscopy) were identified to have both a medial meniscal injury and SCL of the medial femoral condyle. Thirteen horses had both abnormalities identified concurrently, 6 developed a meniscal lesion subsequent to SCL debridement, and 2 developed a SCL subsequent to a medial meniscal injury. Only 4/19 horses were classified as successful and returned to their intended use. The severity of the meniscal injury was significantly associated with the severity of lameness but not with outcome. Conclusions: A low percentage of horses may develop both a meniscal injury and SCL of the medial femoral condyle within the MFT joint and have a poor prognosis. Potential relevance: Trauma to the MFT joint may lead to both meniscal and subchondral bone damage of the medial femoral condyle that may be recognised concurrently or sequentially.     

Meniscal damage diagnosed by ultrasonography in horses: A retrospective study of 74 femorotibial joint ultrasonographic examinations (2000–2005)

This retrospective study reports diagnostic findings in 74 horses with ultrasonographic diagnosis of femorotibial joint damage; it describes the ultrasonographic features of meniscal tears and determines the prevalence of medial or lateral meniscal involvement and of associated synovial effusions. Horses were classified into four groups: with medial meniscal damage, with lateral meniscal damage, with lesions in both menisci, and with no ultrasonographic evidence of meniscal damage. After ultrasonographic appearance, meniscal lesions were described as central degeneration, horizontal tear, partial oblique tear of the distal angle, combined horizontal and oblique tears, or complex tear. Meniscal protrusion or other associated ultrasonographic or radiographic abnormalities were recorded. Of the 74 horses, 54 (73%) had medial meniscal damage, 5 (6.75%) had lateral meniscal damage, 5 (6.75%) had lesions in both menisci, and 10 (13.5%) had no meniscal lesion. Meniscal protrusion occurred in 20 cases (27%). Horizontal tears were the most frequent type of meniscal lesion (26 horses). Complex lesions were found in 6 lateral menisci and 14 medial menisci. Lesions of the cranial meniscal ligaments were seen in 10 horses. Synovial effusion of one or several joint compartments was found in 51 cases (68.9%).This study demonstrates the high prevalence of meniscal tears and synovitis in horses with ultrasonographic evidence of femorotibial derangement. Based on this series of clinical cases, horizontal tears of the medial meniscus appear to be the most frequent soft tissue injury of the equine stifle.     

Acquired peri-articular ganglion cyst in the lateral femorotibial joint in an 18-year-old Percheron cross mare

This report describes a case of sudden lameness in an 18-year-old Percheron cross mare after presumably being kicked by another horse, and the subsequent formation of a ganglion cyst originating from the lateral femorotibial joint. Physical examination, radiographic and ultrasonographic investigation identified a soft round 5 cm diameter mass attached to the left lateral femorotibial joint. After surgical removal, histopathological examination confirmed a ganglion cyst. The horse responded well to the surgical removal of the cyst, and 4 weeks after the surgery, the mare has returned back to her same athletic performance. According to the authors’ knowledge, this is the first reported case that describes a peri-articular ganglion cyst originating from the stifle joint.     

Injuries to the cranial cruciate ligament and associated structures: summary of clinical, radiographic, arthroscopic and pathological findings from 10 horses

The clinical, radiographic, arthroscopic and pathological findings of 10 horses with injury to the cranial cruciate ligament are presented. The most consistent clinical signs included moderate to severe distension of the femoropatellar joint and a Grade III to a Grade V out of V lameness. Craniocaudal instability could be elicited in five horses under general anaesthesia and in one conscious horse. Radiographic evaluation of the stifles revealed that avulsion fracture of the medial intercondylar eminence was the most common finding in six out of 10 horses. Arthroscopic examination of the affected femorotibial joints were performed in five horses. This confirmed the presumptive diagnosis of cranial cruciate ligament injury or rupture. Post mortem examinations were performed on two horses which documented partial tears of the cranial cruciate ligament.     

Digital sheath synovial ganglion cysts in horses

Objectives: To report the clinical features of horses with fluid‐filled masses associated with the digital flexor tendon sheath (DFTS) and outcome after surgery. Study Design: Case series. Animals: Horses (n=10) Methods: Medical records of horses with fluid‐filled masses associated with the DFTS were reviewed and the clinical features, diagnostic methods, treatment, histopathology, and outcome reported. Results: Masses were unilateral (7 hind limb, 3 front limb) and in 8 horses were associated with lameness. In 6 horses, lameness improved by >50% with intrathecal DTFS anesthesia, whereas 2 were less positive but were further improved with perineural anesthesia just proximal to the cyst. Communication between the DFTS and mass was identified in all horses ultrasonographically. Resection of the mass resolved lameness in 7 horses. Histologically (5 specimens), the mass was characterized by a fibrous outer layer without a synovial lining, consistent with a ganglion cyst. Conclusion: The histologic appearance of ganglion cysts suggests that they arise from trauma to the sheath wall and may subsequently be a cause of lameness.     

Arthroscopic Removal of Palmar/Plantar Osteochondral Fragments from the Proximal Interphalangeal Joint in Four Horses

Objective— To describe anatomic considerations and arthroscopic technique in horses for arthroscopic removal of palmar/plantar osteochondral fragments from the proximal interphalangeal (PIP) joint. Study Design— Retrospective study. Animals— Adult horses (n=4) with osteochondral fragments of the palmar/plantar PIP joint. Methods— Arthroscopic removal of palmar/plantar osteochondral fragments within the PIP joint was performed with horses in dorsal recumbency under general anesthesia. Medical records of affected horses were reviewed to determine history; physical, lameness, and radiological findings; surgical technique; complications and outcome. Results— Two horses had lameness localized to the PIP joint. Two other horses had lameness suspected, but not confirmed to the pastern region. One of these horses had a history of intermittent lameness, but was not lame on admission. All horses had radiographic evidence of palmar/plantar osteochondral fragmentation within the PIP joint. Fragmentation was located abaxially in 2 horses in the hind limb and axially in 2 horses in the left forelimb. Osteochondral fragments were successfully removed via a palmar/plantar arthroscopic approach in all horses. Three horses returned to previous levels of athletic performance; 1 horse was used for trail riding instead of reining. Conclusions— Arthroscopy of the palmar/plantar pouch of the PIP joint allowed limited assessment of the joint and removal of osteochondral fragments. Clinical Relevance— Arthroscopy of the palmar/plantar PIP joint pouch for assessment and removal of osteochondral fragments is possible and should be considered when lameness is localized to this joint.     

Cruciate and collateral ligament failure in the equine stifle: seven cases (1975-1985)

Medical records of 7 adult horses with single or multiple ligament failure of the femorotibial ligaments were reviewed to evaluate signalment, history, diagnostic and treatment methods, outcome, and long-term follow-up information. Acute onset of lameness was recorded for most of the horses. Horses with multiple ligament injury had more severe clinical signs than did horses with single ligament injury. Horses with single ligament injury generally required manipulative tests to localize the lameness to the stifle. In all horses, radiography of the stifle assisted in the diagnosis. Horses either were euthanatized (n = 3) or were treated by stall rest (n = 4) after diagnosis. Of the 4 surviving horses, 2 became successful breeding animals, 1 was awaiting breeding, and 1 was used unsuccessfully as a breeding animal. Necropsy findings in 2 of the 3 horses euthanatized after diagnosis revealed early articular damage in 1 horse 1 day after injury, and degenerative joint disease in the other horse 1 year after injury.     

Osteochondral fragmentation of the plantar/palmar proximal aspect of the proximal phalanx in racing horses

SUMMARY The clinical presentation and outcome of treatment is presented for 26 cases of osteochondral fragmentation of the plantar/palmar proximal aspect of the proximal phalanx. Twenty-three were racing Standardbreds and three were racing Thoroughbreds. The most common reason for presentation was an inability to run straight at high speed. Only eight horses presented for lameness, although on examination 19 were lame. A positive flexion test was recorded in 90% of affected fetlock joints and effusion in 48%. Arthroscopic fragment removal was performed on 23 occasions in 21 horses and arthrotomy in one horse. Of the 16 horses that had returned to racing, 12 had improved their performance, while three showed no improvement, and one was retired for other reasons. In three horses refragmentation occurred after surgery, two of which had improved after initial arthroscopic removal. Degenerative changes within the fetlock joint were detected at surgery in eight horses. Of the four horses treated conservatively, one returned to its previous level of performance temporarily after intra-articular medication, one showed no improvement and two were still resting. Plantar/palmar osteochondral fragmentation of the proximal aspect of the first phalanx is a common cause of low-grade lameness in racing horses, and arthroscopic removal results in improvement in race performance in a high percentage of cases.     

Case Report of a Complex Lateral Femorotibial Joint Injury With Popliteal Tendonitis in a Foal

A two-month-old, Spanish breed, foal was referred with right hind limb lameness of two weeks duration. Clinical examinations confirmed a complex combination of lesions within the lateral femorotibial joint, including a subchondral bone cyst in the lateral femoral condyle, a severe popliteal tendonitis, and a delayed cartilage detachment of the lateral femoral condyle. Because of the complex association of these lesions, poor prognosis for an athletic career was given to the owner. Four years later, the filly is pasture sound. Lesions of the popliteal tendon in the horse are uncommon. The popliteal tendon should also be investigated in cases of trauma to the lateral femorotibial joint, as it could be associated with other affected structures.     

Use of magnetic resonance imaging for identifying subchondral bone damage in horses: 11 cases (1999-2003)

OBJECTIVE: To assess the use of magnetic resonance (MR) imaging for identifying subchondral bone damage in the distal limbs of horses. DESIGN: Retrospective study. ANIMALS: 11 horses. PROCEDURE: Medical records of horses with lameness and subsequent evidence of subchondral bone damage as determined by MR imaging were reviewed. Severity and duration of lameness, results of diagnostic local anesthesia and diagnostic testing, surgical and necropsy findings, and treatment were recorded. Outcome was determined by follow-up information obtained from the owner or referring veterinarian. RESULTS: Lameness was localized by physical examination and diagnostic local anesthesia. Lameness was localized to the metacarpophalangeal or metatarsophalangeal joint in 4 horses, distal interphalangeal joint in 5 horses, and tarsocrural joint in 2 horses. The duration of lameness ranged from 2 weeks to 20 months. Magnetic resonance imaging of the affected joints revealed abnormal fluid accumulation within the subchondral bone. None of the abnormalities observed by MR imaging were detected by radiography. Subchondral bone damage was diagnosed in all horses. Arthroscopy of the affected joint was performed in 4 horses. Communication with the articular surface of the affected bone was suspected on the basis of results of MR imaging in 4 horses and was confirmed by arthroscopy in 1 horse and by necropsy in 1 horse. CONCLUSIONS AND CLINICAL RELEVANCE: Magnetic resonance imaging was useful for providing a diagnosis when other imaging techniques did not definitively identify the cause of lameness. Subchondral bone damage was clearly identified by MR imaging and should be considered as a cause of lameness in horses in which radiographic findings are unremarkable.     

Treatment of subchondral cystic lesions of the medial femoral condyle of mature horses with growth factor enhanced chondrocyte grafts: a retrospective study of 49 cases

Reasons for performing study: To evaluate the long‐term clinical outcome after allogeneic chondrocyte and insulin‐like growth factor‐I (IGF‐I) grafting of subchondral cystic lesions (SCLs) of the femoral condyle in horses. Objective: To test the hypothesis that chondrocyte and IGF‐I grafts will improve the long‐term clinical outcome in arthroscopically debrided SCLs. Methods: Medical records of 49 horses with SCLs of the femoral condyle treated by debridement and implantation of chondrocytes and IGF‐I were reviewed. Preoperative radiographs were obtained, and caudocranial radiographic projections were used to establish a ratio between cyst and femoral condyle size. Arthroscopic cyst debridement followed by filling of the bone void with autologous cancellous bone (45 horses) or tricalcium phosphate granules (4 horses) was performed. A paired syringe containing a fibrinogen and chondrocyte mixture in one syringe and calcium‐activated bovine thrombin with IGF‐I in the other was used to cover the surface. A successful outcome was defined as a horse that performed to its intended use without lameness. Results: A successful outcome was achieved in 36 of 49 horses (74%). Preoperative radiography was performed in all horses, with 33 horses having unilateral SCLs of the medial femoral condyle, 15 horses having bilateral SCLs of the medial femoral condyle, and one horse having bilateral SCLs of the lateral femoral condyle. Median age of the horses was 3.3 years. Fifteen horses had preoperative radiographic and arthroscopic evidence of osteoarthritis (OA). A successful outcome was not influenced by age of horse, presence of pre‐existing osteoarthritis or preoperative size of the subchondral cyst. Grafting resulted in success for 80% of horses >3 years old, and in 80% of horses with OA. Conclusions: Implantation of allogeneic chondrocytes supplemented with IGF‐I is an effective treatment for horses with SCLs of the femoral condyle, and particularly for older horses and horses with pre‐existing osteoarthritis. Potential relevance: Chondrocyte implantation may offer a greater chance of long‐term success in older horses and horses with osteoarthritis than has been previously reported with cyst debridement alone.     

Meniscal injury following initial cranial cruciate ligament stabilization surgery in 26 dogs (29 stifles)

OBJECTIVE: To describe clinical signs, arthroscopic findings, and outcome in a group of dogs undergoing second look arthroscopy for the treatment of meniscal tears following original surgery to correct a CCL deficient stifle joint. METHODS: The medical records of 26 dogs from the Veterinary Teaching Hospital at Texas A&M University and the Veterinary Orthopedic Center (Round Rock, Texas) that had second look arthroscopy for lameness following an original surgical procedure were reviewed. Pre-operative clinical findings, 2nd look arthroscopic findings and owner assessed outcome were documented. RESULTS: Postliminary bucket handle tears of the medial meniscus were detected in 22 (75.9%) cases. Other postliminary meniscal injuries included frayed caudal horn tears of the medial meniscus 6 (20.7%), and longitudinal tears of the lateral meniscus 1 (3.4%). An audible or palpable click was present in 27.6% of cases. An improvement or resolution of lameness was reported in 96.5% of cases reported. In conclusion, tears of the medial meniscus are a significant cause of lameness in dogs subsequent to surgery for cranial crucial ligament ruptures. Increased lameness or acute onset of lameness after surgery for cranial crucial rupture is a consistent finding. In rare cases, a palpable or audible click will be appreciated. Arthroscopic evaluation and partial meniscectomy improve or resolve lameness in the majority of cases. CLINICAL SIGNIFICANCE: Sudden or increased lameness in dogs with historical CCL stabilization surgery should be evaluated and treated arthroscopically for postliminary meniscal injury if another cause for lameness can not be determined.     

Osteomyelitis secondary to trauma involving the proximal end of the radius in horses: five cases (1987-2001)

OBJECTIVE: To determine clinical, radiographic, and scintigraphic abnormalities in and treatment and outcome of horses with trauma-induced osteomyelitis of the proximal aspect of the radius. DESIGN: Retrospective study. ANIMALS: 5 horses. PROCEDURE: Data collected from the medical records included signalment; history; horse use; degree of lameness; radiographic, ultrasonographic, and scintigraphic findings; treatment; and outcome. RESULTS: Duration of lameness prior to referral ranged from 14 to 60 days. Mean severity of lameness was grade 3 of 5, and all horses had a single limb affected. All horses had signs of pain during elbow joint manipulation and digital palpation over the lateral aspect of the proximal end of the radius. Radiographic lesions consisted of periosteal proliferation, osteolysis, and subchondral bone lysis. Scintigraphy in 3 horses revealed intense pharmaceutical uptake diffusely involving the proximal end of the radius. Two horses had sepsis of the elbow joint. All horses were treated with antimicrobials long-term; 1 horse was also treated by local perfusion of the radial medullary cavity through an indwelling cannulated screw. At follow-up, all horses had returned to their previous function. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that osteomyelitis of the proximal end of the radius can result from a traumatic injury to the antebrachium. Because lesions may be an extension of septic arthritis, a thorough examination of the wound area and elbow joint is recommended. Prolonged systemic antimicrobial treatment can result in a successful outcome.     

CLINICAL AND DIAGNOSTIC IMAGING CHARACTERISTICS OF LATERAL DIGITAL FLEXOR TENDINITIS WITHIN THE TARSAL SHEATH IN FOUR HORSES

Lateral digital flexor tendonitis is a rarely reported cause of hind limb lameness in performance horses. The purpose of this retrospective study was to describe clinical and diagnostic imaging findings for a group of horses with lateral digital flexor tendinitis within the tarsal sheath. Equine cases with a diagnosis of lateral digital flexor tendonitis and magnetic resonance imaging (MRI) studies of the affected region were retrieved from North Carolina University's medical record database. Recorded data for included horses were signalment; findings from history, physical examination, lameness examination, and all diagnostic imaging studies; treatment administered; and outcome. Four horses met inclusion criteria. Lameness was mild/moderate in severity and insidious in onset in all patients. Responses to flexion tests were variable. All horses showed positive improvement (70–90%) in lameness after tarsal sheath analgesia. Radiographic, scintigraphic, and ultrasonographic findings were inconclusive. For all horses, MRI characteristics included increased T2, PD, and STIR signal intensity within the lateral digital flexor tendon in the area of the tarsal sheath. Tarsal sheath effusion was slight in three horses, and mild/moderate in one horse. With medical treatment, two horses were sound at 6‐month followup, one horse was sound at 1‐year followup, and one horse had a slight persistent lameness (grade 1/5) at 9‐month followup. Findings supported the use of MRI for diagnosing lateral digital flexor tendonitis within the tarsal sheath in horses. Affected horses may have a good prognosis for return to athletic performance following appropriate medical treatment.