Chapter 94: Vestigial Metacarpal and Metatarsal Bones

Anatomy of the Splint Bones

Proximal aspect of the small MC/MT bones articulates with the carpal and tarsal bones, respectively, and provides axial support to these structures

The MC/MT interosseous ligament consists of dense fibrous tissue and is variable among horses, ranging from a pure ligamentous structure to a bony union

A firm fascia covers the tendons in the proximal region of the third metacarpal (MCIII) and metatarsal bone (MTIII) and runs between the small MC/MT bones

A band-like structure extends distally from the distal end of the small MC/MT bones toward the lateral and medial aspect of the condyles of distal MCIII and MTIII

Weight Bearing of MCII vs MCIV

In the forelimb, MCII carries more weight than MCIV because of the flat articulation compared with the oblique configuration of MCIV

MCII articulates with the second and third carpal bone, whereas MCIV only articulates with the fourth carpal bone

Weight Bearing of MTIV

MTIV has only a small articulation with the fourth tarsal bone, providing minimal weight transfer through this articulation

Lateral Dorsal Metatarsal Artery

In the hindlimbs, the location of the lateral dorsal metatarsal artery between MTIII and MTIV makes it particularly vulnerable to accidental or surgical injury

Small MC/MT Bone Fractures

• Kicks from other horses most important cause but may also occur spontaneously during exercise

• One study found that fractures of the MTII and MTIV were more often caused by kicks than fractures of MCII and MCIV and most occurred on pasture or in a permanent group housing system (Donati et al, 2017)

Small MC/MT Bone Fractures - Clinical Examination

• Lameness usually severe in open, proximal fractures and moderate in distal fractures

Small MC/MT Bone Fractures - Diagnostic Analgesia

• High palmar/plantar analgesia will resolve the lameness associated with splint bone fractures, except those involving their articulation

  • In these cases, intraarticular analgesia of the middle carpal joint or tarsometatarsal joint will result in a substantial improvement

What is the most common splint bone involved in proximal fractures?

MTIV

Conservative Treatment of Proximal Splint Bone Fractures

• Standing wound debridement

  • Proximal multifragment splint bone fractures are candidates for conservative treatment

  • Most are open and contaminated so standing wound debridement with removal of loose fracture fragments and temporary installation of a drain should be performed

  • Follow-up radiographs 2-4 weeks later

  • Box stall for 1 month followed by 2 months of hand walking or turnout in a small paddock

  • One study compared conservative and surgical treatment of open multifragment fractures of MTIV and yielded comparable results with respect to return to full work and cosmesis, with conservative therapy being significantly less expensive and associated with less morbidity (Sherlock and Archer, 2008)

  • In another report, 12/14 horses with open multifragment fractures returned to athletic function after conservative therapy (Walliser and Feige, 1993)

  • Another study reported satisfactory healing in the majority of horses that were treated with only wound debridement and no additional stabilization or splint bone removal (Jackson et al, 2007)

Internal Fixation for Proximal Splint Bone Fractures

• No more than the distal 2/3 of the splint bones should be removed with the exception of MTIV

• In cases where more than 2/3 of the splint bone has to be removed, internal fixation of the proximal fragment to MCIII/MTIII with a small plate has been proposed to maintain axial support, especially in cases of MCII fractures

• An experimental study showed that biomechanical alteration in the carpus occurred only when MCII was removed completely, whereas removal of less than or 80% did not alter the biomechanical stiffness (Seabaugh et al, 2012)

  • This suggests that up to 80% of MCII may be removed without altering the biomechanics of the carpus rather than the previously reported 66%

• Preferred technique is plate application in which the screws only engage the small MC/MT bone

  • If the screws inserted through the plate holes cross these bones and penetrate MCIII/MTIII horses often remain lame so the plate will need to be removed 3-4 months post-op

• Appropriate implants include 3.5 mm LCPs or narrow DCPs, semitubular plates, or reconstruction plates

• Ligamentous attachments on the abaxial surfaces of the proximal splint bones make the palmar or plantar abaxial aspect a tension surface, the ideal location for plate application

• Screw fixation alone can be useful in selected cases, such as simple fractures with minimal displacement

  • Generally associated with a relatively high rate of complications such as bone failure, implant failure, radiographic lucency around the screws, and proliferative new bone formation at the ostectomy site

  • In one study only 2/11 horses treated with screw fixation returned to their intended use compared with 6/11 horses treated by plate fixation (Peterson, Pascoe, and Wheat, 1987)

• Post-op external coaptation not needed

• Kept in a box stall for 1 month, followed by walking exercise for an additional 2 months or turnout in a small paddock

Segmental Ostectomy for Proximal Splint Bone Fractures

• Can be performed in some complicated fractures comprised on multiple small fragments

• Linear skin incision made immediately over the affected portion of splint bone

• Contaminated and necrotic tissues debrided and any loose bone fragments, osseous callus, and seqeustra are removed

• Remaining proximal and distal portions of the splint bone are subsequently obliquely severed with an osteotome or an oscillating saw

• In open fractures, a drain is placed for 2-3 days

• Pressure bandages for 2 weeks and horses stall rested for 1 month followed by 2 months of hand walking or small paddock exercise

• Follow-up radiographs recommended to evaluate healing and stability of the proximal fragment

• In one study, three horses with open, proximal multifragment fractures treated with segmental ostectomy returned to normal activity within 8 weeks after surgery with excellent cosmetic results (Jensen et al, 2004)

Removal of the Entire Fourth Metatarsal Bone for Proximal Splint Bone Fractures

• Lateral recumbency with the affected limb up

• Linear skin incision made over the lateral asepect of the entire length of MTIV

• Beginning distally, MTiV and its periosteum are elevated from the surrounding tissues to the site of the fracture

• Avoid the metatarsal artery

• Fracture fragments and the proximal end of MTIV are removed by sharp transection of the proximal ligamentous attachments, including the long collateral and long plantar ligament and the tarsometatarsal articulation

• In open fractures, a drain is placed for 2-3 days

• Because of possible subsequent instability or luxation of the tarsometatarsal joint, a full length hindlimb cast should be considered for recovery and for the first 4 weeks postoperatively

• In one report, 5/8 horses treated with complete removal of MTIV returned to their intended use and were pasture sound (Baxter, Doran, and Allen, 1992)

Prognosis for Proximal Splint Bone Fractures

• Complicated fractures including open, comminuted, and articular fractures have a good prognosis after standing surgical intervention without removal of the distal part of the bone or fixation of the fracture

• Plate fixation recommended only when the proximal part of the splint bone has become unstable relative to MCIII/MTIII

• Prognosis after internal fixation of closed fractures is excellent

Possible Complications of Proximal Splint Bone Fractures

• Possible complications of standing surgical intervention: excessive callus formation, nonunion, instability of the proximal fragment

• Major complication after internal fixation is infection

• Possible complications after segmental ostectomy include instability of the proximal fragment, sequestration of the distal fragment, and excessive exostosis formation at the bone ends

Nonsurgical Management of Midbody Splint Bone Fractures

• Good candidates are horses that have chronic fractures with minimal callus formation and patients with nondisplaced fresh fractures

• Due to motion at the fracture site, healing is often delayed and frequently results in exuberant callus

Surgical Management of Midbody Splint Bone Fractures

• Should be managed surgically with a partial ostectomy

• When fractures are located in the middle third of the bone a partial ostectomy should not cause instability of the remaining proximal portion

• Lateral recumbency

• Incision directly over the affected bone which extends from a few centimeters proximal to the fracture to just distal to the distal aspect or "button"

• Sharp and blunt dissection used to expose the portion of the bone to be excised

• Osteotome and mallet used to transect the splint bone just proximal to the lesion at a 30 to 45 degree angle with the long axis of the bone or an oscillating saw can be used

• Distal part is excised beginning distally and working proximally

• In open fractures drain is placed in the wound for 2-3 days

Prognosis for Midbody Splint Bone Fractures

• Outcome after conservative treatment is favorable, but convalescent time is usually longer

• Prognosis after surgical treatment is very good

• 13 horses with midbody splint bone fractures treated with a segmental ostectomy returned to normal activity within 8 weeks after surgery with excellent cosmetic results

Complications of Midbody Splint Bone Fractures

• Conservative treatment: excessive callus formation leading to secondary suspensory desmitis, nonunion

• Surgical treatment: exostosis formation at the splint bone and/or MCIII/MTIII

Distal Splint Bone Fractures

• Usually occur at the narrowest part of the bone or immediately distal to the distal end of the attachment of the interosseous ligament

• Particularly common in racehorses but can result from external or internal trauma

  • Internal trauma, especially in the forelimb, occurs secondary to excessive loads with extension of the carpus during exercise or secondary to suspensory desmitis where enlargement of the suspensory branch displaces the distal small MC/MT bone abaxially

  • In one study, over 50% of distal small MC/MT bone fractures had an associated suspensory desmitis (Jones and Fessler, 1977)

Nonsurgical Treatment of Distal Splint Bone Fractures

• May be indicated in fractures with minimal or no dislocation

Surgical Management of Distal Splint Bone Fractures

• Surgical excision of the fractured fragment improves the prognosis and reduces the convalescent period

• Incision made directly over the bone, cutting down to the bone surface over the length of the fragment

• Distal tip is grasped with towel clamps or tissue forceps and elevated

• Attachments between the distal fragment and MCIII/MTIII as well as the bandlike structure that extends distal from the distal end of the small MC/MT bone and connects to the suspensory apparatus are severed

• Sharp dissection continued proximal to approximately 8-16 mm proximal to the fracture site

• Small MC/MT bone is transected obliquely with an osteotome or oscillating saw approximately 1 cm proximal to the fracture

• Meticulous hemostasis is indicated to reduce the risk of postoperative excessive new bone formation at the distal stump of the bone

• In open fractures a drain is placed for 2-3 days

Prognosis for Distal Splint Bone Fractures

• Depends on coexistence and degree of suspensory desmitis, sesamoiditis, and/or metacarpophalangeal joint disease

• If no other structures involved, prognosis after surgical removal is excellent

Complications of Distal Splint Bone Fractures

• Conservative: excessive callus formation which could interfere with the suspensory ligament

Splint Bone Exostoses

• Lesions on MCII most common

• Most commonly develop between MCIII and MCII 6-7 cm distal to the carpometacarpal joint

• Possible causes are direct trauma leading to subperiosteal hemorrhage and elevation of the periosteum, instability between MCIII and MCII, MCII fractures, or inflammation and tearing of the interosseous ligament with associated periostitis

  • Inflammation and tearing of the interosseous ligament can results from too much work on hard surfaces or on a circle, especially in immature horses, or from conformation abnormalities (bench knees), or carpus varus deformities

Pathophysiology of Splint Bone Exotoses

• Splints caused by internal trauma are common in younger horses in early training, where the small MC/MT bones are mobile relative to the adjacent MCIII/MTIII and axial forces result in stress and shearing of the interosseous ligament and periosteum

• MCII is most frequently affected because it has the larger articulation with the carpal bones and more extensive soft tissue attachments

• Synostosis between small MC/MT bones and MCIII/MTIII results in stabilization and resistance to strain and shear

Splint Bone Exostoses - Diagnostic Analgesia

• High 4 point block will reduce lameness in horses with splints but this is unspecific

• A selective block (only medial/lateral palmar/plantar metacarpal/metatarsal nerve) provides greater specificity

• Local infiltration of anesthetic over the lesion is an alternative method but may be less effective if a substantial component of the pain is related to inflammation of the interosseous ligament or if suspensory ligament impingement is also a component of the lameness

Nonsurgical Management of Splint Bone Exostoses

• Lameness associated with a clinically active exostosis usually resolves with rest for a period of 2-6 weeks

  • Confined to a stall with exercise limited to daily hand walking or small paddock confinement

• Local cold therapy beneficial to reduce soft tissue swelling in the acute stage

• Local application of DMSO may facilitate reduction of the swelling in the acute and subacute stage

• Infiltration of the exostosis with corticosteroids has a similar effect and will decrease the fibrous and osseous proliferative response

Surgical Management of Splint Bone Exostoses

• Surgery involves a partial ostectomy or surgical debridement of the exostosis with preservation of the bone

• Surgical approach for debridement of large exostosis consists of a shorter incision centered over the lesion

  • After reflecting the soft tissue an osteotome (or chisel) and mallet are used to carefully separate the proliferative periosteal bone from the surfaces of the underlying splint and MCIII/MTIII

• Most important aspect of managing cosmetic splint removals is postoperative bandaging

  • A tight bandage applying direct pressure over the surgery site followed by an outer padded compression bandage should be kept in place for 2-3 weeks to minimize accumulation of blood or serum at the surgical site

• Confined to a stall for 4 weeks after which walking exercise can be performed for an additional 4 weeks, exercise intensity gradually increases during the following 4 weeks

Prognosis for Splint Bone Exostoses

• Prognosis for young horses in early training is generally good if appropriate management is applied

• Splints that develop in mature horses, either as a result of internal or external trauma need more time to heal or the lesion may lead to chronic lameness

• Prognosis following partial ostectomy is fair to good

  • Possible complication is recurrence of the exostosis, periosteum should always be removed with the exostosis to minimize the risk

Polydactyly

• Most frequently described congenital phalangeal malformation in the horse

• In 80% of the cases, the supernumerary digit develops on the medial aspect of the forelimb

• Usually digit represent an atavistic form of a regular small MC/MT bone

• Supernumary MC/MT and digit should be removed to improve the cosmetic appearance and to prevent injury to the digit

• Incision made directly over the supernumary digit starting at the midcannon bone and continuing distal up to the supplementary hoof

• Supernumary digit has extensor and flexor tendons, a suspensory ligament, and sesamoid bones as well as vessels and nerves

• Vessels are ligated and transected together with the tendons as far proximal as possible

• Osteotomy of the digit (usually MCII) is performed at its middle part with an osteotome or oscillating saw