Superficial and Deep Digital Flexor Tendonitis and Suspensory Ligament Desmitis

Superficial digital flexor tendonitis (SDFT), deep digital flexor tendonitis (DDFT), and suspensory ligament desmitis (SLD) are among the most common causes of lameness and premature retirement in athletic horses. These conditions involve inflammation, degeneration, and often partial or complete tearing of the respective soft tissue structures within the metacarpal/metatarsal region. The SDFT is the primary energy-storing tendon of the equine forelimb, subjected to high strains during galloping and jumping. The DDFT, located deeper, is crucial for flexion of the distal interphalangeal joint and is frequently injured in the pastern and foot regions. The suspensory ligament (SL) originates from the proximal palmar metacarpus/metatarsus, inserts on the proximal sesamoid bones, and provides support to the fetlock joint. Injuries range from mild strain (grade 1) to complete rupture (grade 3), with significant impact on performance and soundness. In the equine industry, these injuries are particularly prevalent in Thoroughbred and Standardbred racehorses, as well as in dressage, eventing, and show jumping horses, leading to substantial economic losses and welfare concerns.

The primary etiology of SDFT, DDFT, and SL injuries is biomechanical overload, often occurring during high-speed exercise or repetitive strain. In racehorses, fatigue of the musculotendinous unit leads to excessive strain on the tendon, exceeding its elastic limit and causing microdamage. For the SDFT, this typically occurs during the stance phase of the gallop, when the tendon is maximally loaded. DDFT injuries are often associated with foot imbalance, poor shoeing, or hard ground, leading to excessive tension on the deep digital flexor apparatus. Suspensory ligament desmitis can be caused by hyperextension of the fetlock, especially in horses with poor conformation (e.g., long, sloping pasterns) or in those performing rapid turns and stops (e.g., Western performance horses). Other contributing factors include inadequate conditioning, poor footing, and improper training schedules. Less commonly, direct trauma (e.g., kicks, lacerations) can cause tendon or ligament damage. In some cases, systemic diseases such as equine metabolic syndrome (EMS) or pituitary pars intermedia dysfunction (PPID) may predispose to tendon degeneration due to altered collagen metabolism and impaired healing.

SDFT injuries are most prevalent in Thoroughbred racehorses, with an incidence of 10-30% per racing season. Standardbreds also have a high incidence, particularly of suspensory ligament desmitis. Warmbloods used for dressage and show jumping frequently suffer from DDFT injuries within the hoof capsule. Age is a significant factor; older horses (>10 years) are more prone to chronic degenerative changes, while younger racehorses (2-4 years) often sustain acute injuries due to training intensity. Sex distribution is relatively equal, though some studies suggest a slight male predominance in racing breeds. The forelimbs are more commonly affected than hindlimbs, with the left forelimb being most frequently injured in racehorses due to counterclockwise racing direction. Management factors such as stall confinement with limited turnout, high-concentrate diets, and intense training schedules increase risk. The morbidity is high, with many horses failing to return to their previous level of athletic performance. Mortality is rare but can occur if catastrophic rupture leads to severe lameness and complications.

The pathophysiology of tendon and ligament injuries involves a complex cascade of mechanical, vascular, and cellular events. Initially, excessive strain causes microtears in collagen fibers, leading to disruption of the extracellular matrix. This triggers an inflammatory response, with infiltration of neutrophils and macrophages, release of pro-inflammatory cytokines (e.g., IL-1, TNF-α), and increased vascular permeability. Within days, the tendon enters a reparative phase characterized by proliferation of tenocytes and deposition of type III collagen, which is weaker and less organized than the normal type I collagen. Over weeks to months, remodeling occurs, but the healed tissue often remains biomechanically inferior, with increased cross-sectional area and reduced elasticity. In chronic cases, degenerative changes such as chondroid metaplasia, calcification, and fibrosis may be seen. For the suspensory ligament, similar processes occur, but the ligament's role in supporting the fetlock makes it susceptible to chronic strain, leading to proximal, body, or branch desmitis. In DDFT injuries within the hoof, the tendon may undergo core lesions, which are areas of central necrosis and hemorrhage, often associated with poor vascularity. The presence of adhesions between the tendon and surrounding structures can further impair function.

Intrinsic predisposing factors include conformation: horses with long, sloping pasterns, weak flexor tendons, or upright hoof angles are at higher risk. Age is a factor, as older horses have decreased tendon elasticity and healing capacity. Breed predispositions exist, with Thoroughbreds prone to SDFT injuries, Standardbreds to suspensory desmitis, and Warmbloods to DDFT injuries. Extrinsic factors include training errors such as sudden increases in exercise intensity or duration, inadequate rest periods, and poor footing (e.g., deep, uneven, or hard surfaces). Improper farriery, such as long toes and low heels, increases strain on the DDFT. Stall confinement with limited turnout reduces tendon strength and adaptability. High-grain diets may contribute to systemic inflammation and insulin dysregulation, which can impair collagen synthesis. Environmental factors like cold weather may increase tendon stiffness. Additionally, horses with a history of previous tendon injury are at increased risk of re-injury.

Clinical signs vary depending on the severity and location of the injury. In acute SDFT tendonitis, horses present with sudden-onset lameness, often grade 2-3 on the AAEP scale (0-5). There is heat, swelling, and pain on palpation of the metacarpal region, typically in the mid-metacarpal area. The swelling may be focal or diffuse, and digital pulses are often increased. In severe cases, there may be an obvious defect or gap in the tendon. DDFT injuries within the hoof often cause lameness that is worse on hard surfaces and may be associated with a positive response to hoof testers over the affected area. Suspensory ligament desmitis can cause lameness that is exacerbated by flexion of the fetlock or by lunging on a circle. Proximal suspensory desmitis may present with subtle hindlimb lameness that is difficult to localize. Chronic injuries may show less heat and swelling but persistent lameness and firm, fibrotic thickening of the affected structure. In cases of complete rupture, there is severe, non-weight-bearing lameness with a dropped fetlock and inability to bear weight.

Differential diagnoses for SDFT/DDFT tendonitis and suspensory desmitis include: 1) Fractures of the proximal sesamoid bones, which cause acute severe lameness and are diagnosed by radiography. 2) Osteoarthritis of the fetlock joint, which may present with joint effusion and pain on flexion, diagnosed by intra-articular anesthesia and radiography. 3) Desmitis of the accessory ligament of the superficial digital flexor tendon (inferior check ligament), which causes similar swelling but is localized more proximally. 4) Septic tenosynovitis, which presents with severe lameness, joint effusion, and systemic signs, diagnosed by synovial fluid analysis. 5) Cellulitis or abscess, which may cause diffuse swelling and heat, but is often associated with a wound or skin break. 6) Laminitis, which causes bilateral forelimb lameness and is diagnosed by clinical signs and radiography. 7) Deep digital flexor tendon sheath tenosynovitis, which presents with swelling in the palmar pastern region. 8) Distal interphalangeal joint arthritis, which may cause lameness localized to the foot. 9) Navicular syndrome, which causes chronic forelimb lameness and is diagnosed by nerve blocks and MRI. 10) Stress fractures of the metacarpus/metatarsus, which may cause similar lameness and are diagnosed by scintigraphy or MRI.

The diagnostic approach begins with a thorough history and physical examination, including observation of the horse at rest and in motion. Lameness is graded using the AAEP scale. Palpation of the affected limb is performed to identify heat, swelling, and pain. Diagnostic anesthesia is essential to localize the source of lameness. For forelimb lameness, a palmar digital nerve block is performed first; if the lameness resolves, the foot is implicated. If not, an abaxial sesamoid block is performed, which desensitizes the palmar foot and pastern. A low 4-point block (palmar nerves at the level of the distal metacarpus) is used to rule out the fetlock region. For suspensory desmitis, a high 4-point block (lateral and medial palmar nerves and palmar metacarpal nerves) may be necessary. Ultrasonography is the gold standard for diagnosing tendon and ligament injuries. A 7.5-10 MHz linear transducer is used to evaluate the SDFT, DDFT, and suspensory ligament in both transverse and longitudinal planes. The cross-sectional area, echogenicity, and fiber alignment are assessed. Lesions appear as hypoechoic or anechoic areas. Radiography is useful to rule out bony abnormalities, especially in the foot and fetlock. MRI is indicated for DDFT injuries within the hoof capsule or when ultrasonography is inconclusive. In some cases, scintigraphy may be used to identify stress fractures or increased bone remodeling. Synovial fluid analysis is performed if a septic joint or tendon sheath is suspected.

In uncomplicated tendonitis, laboratory findings are often within normal limits. However, in acute severe injuries, there may be a mild increase in serum muscle enzymes (CK, AST) due to muscle damage. Serum amyloid A (SAA) may be elevated in cases of inflammation or infection. If a septic process is suspected, a complete blood count may show leukocytosis with a left shift, and fibrinogen may be elevated. Synovial fluid analysis from the digital flexor tendon sheath or fetlock joint may reveal increased white blood cell count (>10,000 cells/µL) and total protein (>2.5 g/dL) in septic cases. In chronic cases, there may be no significant laboratory abnormalities. For horses with suspected metabolic disease, baseline ACTH and insulin levels may be measured to rule out PPID or EMS, which can affect healing.

Ultrasonography is the primary imaging modality. In SDFT injuries, the tendon may appear enlarged with a central or diffuse hypoechoic lesion. The cross-sectional area is measured and compared to the contralateral limb. Fiber alignment is assessed in the longitudinal plane; disruption indicates a tear. In DDFT injuries within the hoof, ultrasonography through the heel bulbs may be challenging, and MRI is often preferred. MRI provides detailed images of the tendon within the hoof capsule, allowing identification of core lesions, tears, and adhesions. Radiography is used to evaluate for bony changes, such as enthesiophytes at the insertion of the suspensory ligament or fractures of the proximal sesamoid bones. In cases of chronic suspensory desmitis, radiographs may show sclerosis or lysis at the origin. Scintigraphy can reveal increased bone remodeling in the proximal metacarpal region, which may be associated with proximal suspensory desmitis. Computed tomography (CT) is useful for evaluating the distal limb, especially in cases of complex fractures or when MRI is not available.

Cytology of synovial fluid from the digital flexor tendon sheath or fetlock joint is important to rule out sepsis. Normal synovial fluid is clear, yellow, and has a low white blood cell count (<1,000 cells/µL) and total protein (<2.5 g/dL). In septic tenosynovitis, the fluid is turbid, with a WBC count >30,000 cells/µL and total protein >4.0 g/dL, and bacteria may be seen on Gram stain. Histopathology of tendon biopsies is rarely performed clinically but may be used in research. In chronic injuries, histology shows disorganized collagen fibers, increased cellularity, and areas of fibrosis or calcification. In acute injuries, there is hemorrhage, edema, and inflammatory cell infiltration.

Treatment of SDFT, DDFT, and suspensory ligament injuries depends on the severity and chronicity. For acute injuries, the RICE protocol (Rest, Ice, Compression, Elevation) is initiated. Cold therapy (ice water or ice packs) is applied for 20-30 minutes every 4-6 hours for the first 48-72 hours. The limb is bandaged to reduce swelling. Non-steroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone (2.2-4.4 mg/kg IV or PO q12-24h) or flunixin meglumine (1.1 mg/kg IV or PO q12-24h) are administered for pain and inflammation. In severe cases, a controlled exercise program is implemented after the acute phase. Surgical options include tendon splitting (for core lesions) or desmotomy of the accessory ligament. Intralesional therapies such as platelet-rich plasma (PRP), stem cell therapy, or bone marrow aspirate concentrate are increasingly used to enhance healing. Extracorporeal shockwave therapy (ESWT) may be beneficial for suspensory desmitis. For DDFT injuries within the hoof, corrective farriery is essential, often involving a rolled-toe shoe and heel elevation to reduce tension on the tendon. In chronic cases, a graduated rehabilitation program is crucial, starting with hand-walking and progressing to trotting and cantering over 6-12 months. In cases of complete rupture, surgical repair may be attempted, but the prognosis is poor.

The prognosis for return to athletic function varies. For SDFT injuries, the prognosis is guarded; approximately 60-70% of racehorses return to racing, but many do not return to their previous level. The prognosis is worse for severe lesions (grade 3) or those involving a large cross-sectional area (>50%). For DDFT injuries within the hoof, the prognosis is poor to fair, with only 30-50% of horses returning to soundness. Suspensory ligament desmitis has a better prognosis for non-racing disciplines, but for racehorses, the prognosis is guarded. Negative prognostic indicators include bilateral injuries, chronicity, and the presence of adhesions. Early diagnosis and appropriate treatment improve the chances of a successful outcome.

Follow-up is essential to monitor healing and prevent re-injury. Serial ultrasonography is performed every 4-8 weeks to assess the tendon's cross-sectional area, echogenicity, and fiber alignment. The horse's lameness is re-evaluated regularly. A structured rehabilitation program is implemented, with gradual increases in exercise duration and intensity. Farriery should be maintained every 4-6 weeks to ensure proper hoof balance. In horses returning to athletic work, a recheck ultrasound is recommended before resuming full training. For horses with DDFT injuries, MRI may be repeated at 6-12 months to assess healing. Long-term follow-up is recommended to monitor for chronic changes and prevent re-injury.

Pearls: Always perform diagnostic anesthesia to localize the lameness before imaging. Use a high-frequency linear transducer for optimal tendon imaging. In acute injuries, early cold therapy and NSAIDs are crucial. Consider intralesional therapies such as PRP for core lesions. For suspensory desmitis, a high 4-point block is essential for diagnosis. Pitfalls: Do not overlook the possibility of a DDFT injury in the foot; MRI is often necessary. Avoid premature return to work, as re-injury is common. Do not rely solely on radiography; ultrasonography is essential for soft tissue evaluation. In cases of severe lameness, always rule out fractures or sepsis before treating for tendonitis.

For acute tendonitis, phenylbutazone (4.4 mg/kg IV or PO q12h for 1-2 days, then 2.2 mg/kg q12h) is commonly used. Flunixin meglumine (1.1 mg/kg IV or PO q12h) may be used for more severe pain. For chronic pain, firocoxib (0.1 mg/kg PO q24h) is an option. In cases of suspected sepsis, broad-spectrum antibiotics such as penicillin G (22,000 IU/kg IV q6h) and gentamicin (6.6 mg/kg IV q24h) are administered. For intralesional therapy, platelet-rich plasma (PRP) is prepared from autologous blood and injected into the lesion under ultrasound guidance. Stem cell therapy (bone marrow-derived mesenchymal stem cells) is also used. Extracorporeal shockwave therapy (ESWT) is applied at 2000-3000 shocks at 0.15-0.25 mJ/mm². For DDFT injuries, corrective shoeing is more important than medication. In all cases, a controlled exercise program is essential.

Numerous studies have evaluated the efficacy of various treatments for tendonitis. A landmark study by Smith et al. (2003) demonstrated that intralesional injection of bone marrow-derived mesenchymal stem cells improved tendon healing in a horse model. Another study by Dyson et al. (2004) found that PRP treatment resulted in better fiber alignment and reduced re-injury rates compared to controls. A meta-analysis by O'Meara et al. (2010) concluded that ESWT is beneficial for suspensory desmitis. The AAEP consensus statement on tendon injuries recommends a multimodal approach, including controlled exercise, intralesional therapies, and corrective farriery. However, there is a lack of large randomized controlled trials, and many recommendations are based on expert opinion and small case series.