Jenson PV Review 0503

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Comments? Questions? Email: [email protected] Web: VetLearn.com • Fax: 800-556-3288 CE Article #6 (1.5 contact hours) Refereed Peer Review Splint Bone Disorders in Horses KEY FACTS Kansas State University I Treatment options for splint Paul W. Jenson, DVM bone disorders are affected by Earl M. Gaughan, DVM, DACVS the fact that metacarpals II and James D. Lillich, DVM, MS, DACVS IV provide a greater support function to the carpus than James E. Bryant, DVM, DACVS metatarsals II and IV provide to the tarsus. ABSTRACT: Splint bone disorders in horses include exostoses and fractures in distal, middle, and proximal anatomic locations. The function of each splint bone depends on the size of the I Proximal splint fractures may proximal articulation and the supporting soft tissue structures. Exostoses develop in response require internal fixation to to inflammation but seldom cause lameness unless the inflammatory cause persists or sus- preserve normal carpal or tarsal pensory ligament function is altered. Acute splint bone exostoses can be successfully treated function. with antiinflammatory drugs and rest, and chronic exostoses may require surgical treatment. Treatment options for splint bone fractures vary with the character and location of the fracture I Closed middle and distal and include conservative management, surgical resection, and internal fixation. Surgical fractures can usually be resection options for open splint bone fractures include segmental resection, which preserves the proximal and distal segments of the splint bone. The prognosis for splint bone disorders is managed conservatively. generally good with timely diagnosis and treatment.

isorders of the small metacarpal, metatarsal, or splint bones occur in horses of all ages. The distal and exposed locations of the splint bones render Dthem prone to external trauma; however, injury also occurs with no history or evidence of external trauma, which correlates with biomechanical trauma from the forces of weight bearing and exercise.1–4 This article discusses exostoses and fractures, which are described as proximal, midshaft, and distal and are characterized according to severity and association with adjacent structures. Treatment options vary considerably with the location and severity of the lesion, but the prognosis is generally good for horses with appropriately managed splint bone injuries. NORMAL ANATOMY Consideration of anatomic relationships can be important in evaluating injury and formulating treatment plans, which vary according to the structural function of each splint bone. The splint bones, together with the cannon bone, form the skeletal structures of the metacarpus and metatarsus. The articular surfaces and proximal aspects of the splint bones provide structural support for the carpus and tarsus. Metacarpal splint bones have larger articular surfaces and more soft tissue attachments than metatarsal splint bones, indicating a greater weight support function in the forelimbs. The second metacarpal bone may articulate with the second carpal bone alone or with the second and third carpal

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bones combined.5,6 The fourth metacarpal bone articu- Diagnosis lates only with the fourth carpal bone. The second Clinical signs associated with splint exostoses can be metatarsal bone articulates with the fused first and variable. Chronic exostoses are not usually painful; how- second tarsal bones, and the fourth metatarsal bone ever, pain can be elicited on palpation, and lameness may articulates with the fourth tarsal bone. The second be acutely present after trauma. Exostoses on the axial metatarsal is typically smaller in diameter than the surface of the splint bone have been referred to as blind fourth metatarsal, but the articular surface of the splints and may not be readily detected; however, lameness fourth metatarsal is quite small in relation to the large may be present if new bone impinges on the suspensory proximal end.7 ligament.8 Proximal exostoses (“knee splints”) may be The soft tissue attachments on the proximal aspects painful and may predispose patients to carpal/tarsal arthri- of the splint bones provide a tension force that supports tis.7 Mineralization of the interosseous ligament has been the compressive forces exerted on the carpal and tarsal called “true splint” and is not typically a source of pain.1,8,12 bones. The second metacarpal bone is attached proxi- Diagnosis of splint exostoses depends on physical mally to the medial collateral carpal ligament, flexor examination and lameness evaluation, including local carpi radialis, and extensor carpi obliquus. The fourth anesthesia, and is confirmed with diagnostic imaging. metacarpal bone is attached proximally to the lateral Radiography can define lesion location and the presence collateral ligament of the carpus, accessory carpo- of associated fracture, sequestra, or arthritis. Standard metacarpal ligaments, and ulnaris lateralis tendon of four-view studies and additional oblique projections insertion. Proximal metatarsal soft tissue support may be necessary to completely identify the location includes the medial collateral tarsal ligament and and extent of the proliferative new bone (Figure 1). medial tendon of the cranial tibial muscle, which attach to the proximal second metatarsal bone, as well as the long lateral collateral ligament and long plantar ligament originating on the plantar tuber calcis, which attach to the proximal fourth metatarsal bone.8,9 The interosseous ligament attaches the proximal two- thirds of each splint bone to the cannon bone. Liga- ment fibers originate on the caudal surface of the third metacarpal/metatarsal and run in a caudoproximal direction to insert on the cranial surface of the splint bones.10 The interosseous ligament stabilizes the splint bones from tension and torsion forces exerted by proximal soft tissue attachments. As a horse ages, the interosseous ligament loses pliability and can mineral- ize. Les et al6 reported some degree of metacarpal fusion in 192 of 200 metacarpi in racing thoroughbreds and found that fusion of the second metacarpal to the third metacarpal is more common than fusion of the fourth metacarpal to the third metacarpal. EXOSTOSES An exostosis is defined as a benign new growth pro- jecting from a bone surface.11 Exostoses of the second and fourth metacarpal/metatarsal bones have been referred to as splints, and in young horses, they are associated with the onset of training, conformation abnormalities (i.e., “bench knees”), improper hoof care, and nutrition imbalances.1,5,7 The inner cambial layer of the periosteum is active in young horses and tends to have an osteogenic response to inflammation. A contributing Figure 1— factor to periostitis and periosteal proliferation can be An oblique radiograph of the second and third metacarpals demonstrates bone production characteristic of a interosseous desmitis. Exostoses can also form secondary splint exostosis. to blunt trauma, fracture, or sequestrum formation.

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Other imaging modalities can further define splint horses are the left medial and right lateral metatarsal lesions and associated soft tissue injury. Scintigraphy bones.1–3 These fractures may result from increased may reveal inflammation in the soft tissues and bone.13 axial compression in thoroughbred, or ligament and Computed tomography can delineate subtle anatomic fascial tension in standardbred, racehorses racing in a lesions.14 Ultrasonography of the suspensory ligament counterclockwise direction.1–3 Fractures of the distal can help determine the size of axial exostoses and the metacarpal and metatarsal bones were a consistent extent of associated suspensory desmitis.1,8,15 finding with in vitro fetlock hyperextension strain application during strength testing of the forelimb sus- Treatment pensory apparatus.4 The goal of treating acute splint bone exostoses is reduction of inflammation and bone reactivity. Rest Diagnosis appears to be critical for success. NSAIDs, such as Clinical signs of distal splint bone fractures include phenylbutazone (2.2 to 4.4 mg/kg PO sid or bid), can mild lameness, local swelling, and pain on palpation. reduce inflammation, periosteal reaction, and pain. Direct digital pressure on the skin over a fractured Topical dimethyl sulfoxide preparations, systemic 10% splint bone may cause increased lameness, and local dimethyl sulfoxide (IV or PO), cold-water therapy or anesthesia may help isolate the source of pain.18 Con- ice packing, and perilesional corticosteroid injections current inflammation of the suspensory ligament and may also decrease local inflammation.1,7,16 soft tissues of the fetlock joint is not uncommon. Counter-irritants (e.g., pin firing, freeze firing, scle- Diagnosis of splint bone fractures (proximal, mid- rosing agents, internal and external blistering) have shaft, or distal) usually depends on a complete physical been and are still used to treat splint exostoses; however, examination and radiography. Nondisplaced fractures the efficacy of these treatments has been questioned.1,7,16 may not be radiographically visible until after bone Counter-irritants theoretically function to convert low- resorption and remodeling begins, which may take 7 to level inflammation to active, acute inflammation and 14 days. Additional diagnostic aids for acute fractures promote more rapid osteogenesis; however, success has include infrared thermographic imaging and nuclear not been documented by controlled evaluation.10 Surgical treatment for simple exostoses is not indicated acutely after injury. Remodeling of acutely injured bone may be a desired healing response; however, resection of proliferative bone may be indicated at a later date to improve cosmesis or remove axially oriented bone that interferes with suspensory ligament function.12,15 Prognosis The prognosis for return to athletic function after exostosis formation or removal is good. Concurrent suspensory desmitis or arthritis of the distal carpus/tarsus worsens the prognosis. Recurrence is the major compli- cation after surgical excision of splint exostoses. Barber et al12 reported an excellent clinical appearance after surgical removal of splint bone exostoses; however, exostoses that develop in response to conformation abnormalities may have a reduced likelihood of acceptable cosmetic healing. DISTAL THIRD SPLINT BONE FRACTURES The most common fracture site in the splint bone is the distal third. Distal fractures are typically closed and can involve little or no evidence of external trauma. The average age of horses with distal fractures is reportedly 6 years.8,17 The left lateral and right medial metacarpal bones are the most common sites of distal splint bone fracture in thoroughbred horses.1,2 The most common sites of distal splint bone fracture in standardbred 386 Equine Compendium May 2003

scintigraphy. The soft tissue phase of nuclear scintigra- fractures usually require surgical treatment. Resection phy may help evaluate suspensory ligament inflamma- of the portion of a splint bone distal to an open mid- tion, and the bone phase may assist in diagnosing shaft fracture site has historically been recom- nondisplaced fractures.13 Ultrasonography of the sus- mended.8,20 However, segmental resection of the pensory ligament may be indicated with concurrent traumatized midshaft has been successful, leaving the suspensory desmitis.1 Culture and sensitivity testing is distal segment of the splint bone undisturbed21–23 (Figures indicated for all open fractures. 2A and 2B). Treatment Prognosis Rest and administration of NSAIDs are the treat- The prognosis for horses with midshaft splint bone ments of choice for closed distal splint bone fractures. fractures is good. Segmental resection may be less invasive Verschooten et al19 reported an 80% incidence of and may reduce morbidity associated with a more spontaneous healing of distal splint bone fractures. extensive incision and resection of longer portions of Concurrent suspensory desmitis is often the cause of the fractured splint bone.22 lameness; therefore, early decisions for resection of the distal splint bone may not be indicated. Surgical PROXIMAL THIRD SPLINT BONE FRACTURES removal of the distal fragment is indicated with open Proximal splint bone fractures often result from displaced fractures or if the fracture is determined to external trauma and are frequently open. be the cause of chronic lameness. Surgi- cal resection should be performed with aseptic technique under general anesthesia. However, a simple distal splint ostectomy using sedation and local anesthesia can be performed with the horse standing.15 Prognosis The prognosis for soundness is good for horses with distal splint bone fractures. Suspensory desmitis affects the prognosis and is the most likely limiting factor for return to exercise. Periosteal proliferation at a fracture or resection site can affect cosmetic results but does not usually affect function. MIDDLE THIRD SPLINT BONE FRACTURES Midshaft fractures of the small metacarpal/metatarsal bones commonly result from external trauma. Diagnosis Clinical signs include mild to mod- erate lameness and local signs of inflammation. An open wound and/or AB draining tract is often present and may indicate sequestrum formation. Figure 2A—Midshaft splint fracture Figure 2B—Postoperative results with a sequestrum Treatment Figure 2— Closed, nondisplaced midshaft frac- (A) A dorsolateral–plantaromedial oblique radiograph indicating a midshaft splint fracture with a sequestrum. (B) Postoperative results of a segmen- tures heal well with stall rest and tal resection of fragments of the midshaft splint fracture in Figure 2A. NSAID administration. Open midshaft

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Diagnosis Clinical signs can vary due to the association of the carpal and tarsal joints and the proximal soft tissue attachments. Lameness can be mild to severe depending on fracture severity, articular stability, and articular sepsis. Instability of the fracture and the articular surface can result from soft tissue distraction forces, which may displace proximally the distal end of the proximal fragment. Proximal metacarpal splint fractures may have concurrent effusion of the distal carpal joints because of communication between the intercarpal and carpometacarpal joints. In contrast, proximal metatarsal splint fractures seldom cause palpable effusion in the distal tarsal joints, even in the presence of sepsis. Diagnostic evaluation of proximal splint fractures should include examinations of the distal carpal or tarsal joints. A complete radiographic examination should include the carpus/tarsus. Potentially contaminated joints near an open wound should be distended with a sterile solution from a site distant to the wound to determine articular communication.24 Contrast radiography may also be used to determine articular communication with an open wound. Early determination of joint involve- ment is essential for timely, appropriate treatment. Proximal Metacarpal Fractures Treatment Successful management of open articular fractures of the proximal metacarpal splint bones depends on early diagnosis and treatment. The distal carpal joints should Figure 3—A DLPM radiograph indicating a proximal articular be treated with intra-articular lavage and antibiotics fracture of the fourth metatarsal. when contaminated. Infected bone and soft tissue should be debrided and appropriate antibiotic therapy initiated after tissue sampling for culture and sensitivity splint bone may need to be fixed in position.15 How- testing. Unstable articular fractures may require inter- ever, large segments of splint bones have been resected nal fixation. Delayed fracture repair and wound closure without a resultant need for stabilization with orthopedic may be necessary to decrease bacterial numbers in implants. severely contaminated wounds.8,20,25,26 Intensive local wound treatment may be necessary for 3 to 7 days Prognosis before a fracture is repaired.25 The prognosis for open or closed proximal Nonarticular proximal metacarpal splint bone frac- metacarpal splint bone fractures is good with early tures may require internal fixation and debridement if reduction, fixation, and rest. The prognosis is guarded the fracture is open. Unstable fractures of the proximal when chronic infectious arthritis or osteomyelitis is portion of the second metacarpal bone, whether open present.17,25 or closed, usually require repair to provide stability for the large articular surface.8,15,25 However, fractures with Proximal Metatarsal Fractures intact supportive soft tissues may not have substantial Treatment fragment displacement, and repair with implants may Open and/or comminuted fractures of the fourth not be required. metatarsal can successfully heal with stall confinement Open fractures with septic osteomyelitis or large and wound management (Figure 3). Treatment should sequestra require resection of infected bone. Removal of be initiated within 48 hours of trauma for the best out- more than 50% of the medial metacarpal bone can come.25 Extensive debridement, including amputation result in carpal instability, and the remaining proximal of the entire distal portion of the splint bone, may be

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needed. Removal of the entire body of the fourth 15. Richardson DW: The metacarpal and metatarsal bones, in Auer metatarsal has been successfully performed.27 Complete JA (ed): Equine Surgery. Philadelphia, WB Saunders, 1999, pp 810–821. removal of the fourth metatarsal may be indicated for 16. Rose RJ, Hodgson DR: Manual of Equine Practice. Philadelphia, severe comminution, crushed bone with existing infec- WB Saunders, 1993, p 94. tion, or fractures that do not respond to conservative 17. Peterson PR, Pascoe JR, Wheat JD: Surgical management of proxi- 15,25,27 management. This has resulted in five of seven mal splint bone fractures in the horse. Vet Surg 16:367–372, 1987. 27 horses returning to athletic function. 18. Dyson S: Some observations on lameness associated with pain in Internal fixation is recommended for severely dis- the proximal metacarpal region. Equine Vet J Suppl Sept. placed or articular metatarsal splint bone fractures.25 (6):43–52, 1988. However, closed and minimally displaced proximal 19. Verschooten F, Gastuys F, De Moor A: Distal splint bone fractures in the horse: An experimental and clinical study. Equine fourth metatarsal fractures can heal with stall confine- Vet J 16(6):532–536, 1984. ment alone.21 20. Doran R: Fractures of the small metacarpal and metatarsal (splint) bones, in Nixon AJ (ed): Equine Fracture Repair. Prognosis Philadelphia, WB Saunders, 1996, p 206. The prognosis for proximal second and fourth 21. Richardson DW: Fractures of the small metacarpal and metatarsal metatarsal fractures is good. Fractures of the fourth bones, in White NA, Moore JN (eds): Current Practice of Equine metatarsal are reported to have a good prognosis, even Surgery, ed 1. Philadelphia, JB Lippincott, 1990, pp 636–641. with chronic infection.20,27 22. Gaughan EM: Splint bone fractures in horses. Proc Am Coll Vet Surg:171–173, 2000. 23. Jenson PW, Gaughan EM, Lillich JD, et al: Segmental ostectomy REFERENCES of splint bones in horses: 17 cases (1993–2002), submitted for 1. Stashak TS: Adams’ Lameness in Horses, ed 4. Philadelphia, Lea publication, 2003. & Febiger, 1987, pp 615–622. 24. Gaughan EM: Wounds of tendon sheaths and joints in horses. 2. Bowman KF, Evans LH, Herring ME: Evaluation and surgical Compend Contin Educ Pract Vet 16(4):517–529, 1994. removal of fractured distal splint bones in the horse. Vet Surg 25. Baxter GM: Management of proximal splint bone fractures in 11(4):116–120, 1982. horses. Proc Annu Conv Assoc Equine Pract:419–427, 1992. 3. Jones RD, Fessler JF: Observations on small metacarpal and 26. Bowman KF, Fackelman GE: Surgical treatment of complicated metatarsal fractures with or without associated suspensory fractures of the splint bones in the horse. Vet Surg desmitis in standardbred horses. Can Vet J 18:29–32, 1977. 11(4):121–124, 1982. 4. Bukowieki CF, Bramlage LR, Gabel AA: In vitro strength of the 27. Baxter GM, Doran RE, Allen D: Complete excision of a fractured suspensory apparatus in training and resting horses. Vet Surg fourth metatarsal bone in eight horses. Vet Surg 21(4):273–278, 16:126–130, 1987. 1992. 5. Rooney JR: Biomechanics of Lameness in Horses. Baltimore, Williams & Wilkins, 1969, pp 150–154. 6. Les CM, Stover SM, Willits NH: Necropsy survey of metacarpal fusion in the horse. Am J Vet Res 56(11):1421–1432, 1995. ARTICLE #6 CE TEST 7. Goble DO: The small metacarpal and metatarsal bones, in The article you have read qualifies for 1.5 con- Mannsman RA, McAllister ES (eds): Equine Medicine and tact hours of Continuing Education Credit from Surgery, ed 3, vol 2. Santa Barbara, CA, American Veterinary the Auburn University College of Veterinary Med- Publications, 1982, pp 1115–1120. CE 8. Ray C, Baxter GM: Splint bone injuries in horses. Compend icine. Choose the best answer to each of the follow- Contin Educ Pract Vet 17(5):723–731, 1995. ing questions; then mark your answers on the 9. Ghoshal NG: Sisson and Grossman’s The Anatomy of the Domestic postage-paid envelope inserted in Compendium. Animals. Philadelphia, WB Saunders, 1975, p 593. 10. Oridge RM: The equine metacarpus. Part 1: The splint bones, in Grunsell CSG, Hill FWG (eds): Veterinary Annual, issue 24. Bristol, UK, John Wright & Sons, 1984, pp 155–163. 1. Chronic exostoses a. are usually painful. 11. Blood DC, Studdert VP: Bailliere’s Comprehensive Veterinary Dictionary. London, Bailliere Tindall, 1990, p 344. b. are not usually painful but may cause lameness if suspensory ligament function is altered. 12. Barber SM, Caron JP, Pharr JW: Surgical removal of metacarpal/ metatarsal exostoses. Proc Am Assoc Equine Pract:371–383, c. require surgical management. 1986. d. are diagnosed with ultrasonography. 13. Ehrlich PJ, Dohoo IR, O’Callaghan MW: Results of bone scintigraphy in racing standardbred horses: 64 cases (1992–1994). 2. Acute exostoses JAVMA 215(7):982–990, 1999. a. require surgical management. 14. Tucker RL, Sande RD: Computed tomography and magnetic b. can be managed with antiinflammatory drugs. resonance imaging of the equine musculoskeletal conditions. Vet c. do not cause lameness. Clin North Am Equine Pract 17(1):145–157, 2001. d. resolve with the onset of training.

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3. Distal splint bone fractures a. typically show no evidence of external trauma. b. typically show evidence of external trauma. c. do not cause lameness. d. are not associated with suspensory ligament function.

4. Proximal splint bone fractures a. do not affect weight-bearing structures. b. may result in articular instability or sepsis. c. are frequently closed fractures. d. cause palpable effusion in the distal tarsal joints.

5. Midshaft splint bone fractures a. are often associated with a draining tract. b. cause severe lameness. c. do not result from external trauma. d. are not associated with open wounds.

6. Splint bone fractures a. are definitively diagnosed with a physical examination. b. are definitively diagnosed with local anesthesia. c. are always radiographically apparent. d. may require 7 to 14 days for bone resorption and remodeling to be radiographically visible.

7. Distal splint bone fractures a. require surgical ostectomy. b. reportedly have an 80% incidence of spontaneous healing. c. do not usually heal with conservative management (i.e., rest and NSAIDs). d. are not associated with suspensory desmitis.

8. Midshaft splint bone fractures a. can be surgically managed with segmental resection. b. usually heal with rest and NSAIDs. c. require complete distal splint bone ostectomy. d. are not associated with sequestered bone.

9. Proximal metacarpal splint bone fractures are often open and a. may need intensive wound treatment for several days before fracture repair. b. do not need to be debrided. c. do not need to be stabilized. d. should optimally be stabilized with lag screws in the cannon bone.

10. Proximal metatarsal fractures a. do not heal with stall confinement and wound management. b. do not require internal fixation. c. often require extensive debridement. d. have a poor prognosis.