Anatomy

The metatarsal region is bordered by the tarsometatarsal joint proximally and the metatarsophalangeal joint distally. The large third metatarsal bone (MtIII) provides all structural support and weight bearing and articulates predominantly with the third tarsal bone proximally and the proximal phalanx and proximal sesamoid bones (PSBs) distally. A prominent nutrient foramen in the MtIII should not be mistaken for a fracture and is usually located slightly higher than that of the third metacarpal bone (McIII).¹

The second metatarsal bone (MtII) and fourth metatarsal bone (MtIV) are commonly referred to as the medial and lateral splint bones, respectively. The MtII articulates with the combined first and second tarsal bones proximally and ends distally with an enlarged “bell.” The MtIV articulates with the fourth tarsal bone proximally but transmits less load than the MtII. The dorsal metatarsal artery runs obliquely, in a distoplantar direction, in the proximal lateral aspect of the metatarsal region and then parallel and close to the dorsal aspect of the MtIV as it courses distally in the midmetatarsal region. The dorsal metatarsal artery then courses deep to the MtIV and must be avoided during distal splint ostectomy. I examined one horse in which the dorsal metatarsal artery coursed through a bony and fibrous ringlike foramen in the distal aspect of the MtIV and was inadvertently severed during ostectomy of the MtIV. Because the dorsal metatarsal artery is superficial, it can be lacerated from wounds in the lateral metatarsal region. Bleeding can be profound, and the ends of a lacerated dorsal metatarsal artery should be ligated unless anastomosis is performed with the horse under general anesthesia. Collateral circulation develops if ligation is necessary. The long plantar ligament attaches to the proximal aspect of the MtIV.

The orientation of the metatarsal bones is clinically important. The proximal aspect of the MtIV is large and is located in a more plantar location than its counterpart, the fourth metacarpal bone (McIV). This orientation makes it impossible to palpate the normal proximal and midbody portions of the suspensory ligament (SL). Therefore unless it is grossly enlarged, the SL cannot be palpated, and conditions such as mild-to-moderate suspensory desmitis can easily be missed, particularly if lameness is chronic and signs of acute inflammation are not present.

Dense metatarsal fascia attaches to the abaxial margins of the MtII and the MtIV and encircles the SL, deep digital flexor tendon (DDFT), and superficial digital flexor tendon (SDFT). A thinner fascia, the suspensory laminar fascia, lies superficial to the SL, deep to the DDFT. The metatarsal and laminar fascial layers appear confluent at the attachment on the MtII. Swelling of the SL within these fascial compartments can cause pain by compression of the adjacent metatarsal nerves.² The SL has a broad origin on the proximal, plantar surface of the MtIII and a small attachment to the distal tarsal bones. The hind SL is slightly longer and thinner than the front SL, and muscle content may be higher in hindlimbs compared with forelimbs, which can complicate ultrasonographic examination.³ It has been suggested that the higher incidence of suspensory desmitis in Standardbreds (STBs) than in Thoroughbreds (TBs) is related to higher muscle content, but this is unproven.³

The plantar surface of the MtIII and the origin of the SL are oblique and not parallel to the dorsal surface of MtIII. During ultrasonographic examination, care should be taken to account for obliquity and to position the transducer perpendicular to SL fibers (see Chapter 72). The SL is bordered by the MtII and the MtIV and is thus trapped within a bony and dense soft tissue encasement. In the midmetatarsal region the body of the SL can be palpated as it emerges from this bony encasement and courses distally to divide into the medial and lateral branches that attach to the abaxial border of each PSB. The accessory ligament of the DDFT (ALDDFT) is usually present and is a variably sized structure ranging from thin to thick and occasionally has two or three parts; in a small proportion of horses (10 of 165, or 6.1%) there was no identifiable ALDDFT.⁴ Although the ALDDFT is anatomically present in most horses, injury is rare (see Chapter 71). Desmotomy to manage flexural deformity is rarely performed and generally would be ineffective because of the small size of the ALDDFT. I examined an aged Quarter Horse gelding with enlarged ALDDFTs with unusual flexural deformity characterized by intermittent, dynamic heel elevation; during walking the heels would land flat, but during standing the heels elevated bilaterally (see Chapter 71). I severed the ALDDFT in each hindlimb, and although improvement was seen temporarily, flexural deformity recurred. The DDFT is located plantar to the SL for most of the metatarsal region, but in the proximal metatarsal region it is positioned plantaromedially. The SDFT is located plantar to both the SL and DDFT. Because the tarsal sheath ends in the proximal metatarsal region, distention can sometimes be palpated (see Chapter 76). The digital flexor tendon sheath (DFTS) begins in the plantar, distal third of the metatarsal region and encompasses the DDFT and SDFT (see Chapter 74).

In the proximal third of the metatarsal region the lateral digital extensor tendon joins with the long digital extensor tendon, and the combined tendons course distally. The fibularis tertius and cranialis tibialis attach to the proximal, dorsal, and medial aspects of the MtIII. Osseous proliferation and fracture can occur at these locations.

Distal continuation of the superficial fibular and saphenous nerves provides dorsal skin sensation. The tibial nerve divides to form the medial and lateral plantar nerves in the plantar tarsal region. The lateral plantar nerve, which courses plantar to the origin of the SL, gives off a deep branch that innervates the proximal aspect of the SL and gives off branches, which continue distally as the lateral and medial plantar metatarsal nerves that course on the axial aspects of each respective splint bone (see Figure 10-22; Figure 43-1). Neurectomy of the deep branch of the lateral plantar nerve provides analgesia of the origin of the SL and the plantar cortex of the MtIII and partial analgesia of sites more distal to the level of and including the metatarsophalangeal joint. Accurate interpretation of diagnostic analgesia is critical when planning this surgical procedure. Neurectomy has received considerable attention recently as a surgical approach for management of proximal suspensory desmitis (PSD; see later).^5,6 Histological changes consistent with nerve compression have been identified in horses undergoing neurectomy of the deep branch of the lateral plantar nerve, and nerve compression was proposed as a possible cause of residual pain in horses even after desmitis resolved.⁶ In that study, 62% of horses returned to soundness after neurectomy,⁶ whereas in a different study 19 of 20 horses treated by neurectomy and laminar fasciotomy returned to the previous level of performance.⁵ The deep branch of the lateral plantar nerve that was examined histologically⁶ lies within the dense metatarsal but outside the laminar fascial planes. Recognition of neuritis in this segment indicates that compression of the deep branch may be occurring within this fascial compartment. The lateral plantar nerve and its counterpart, the medial plantar nerve, continue distally between the SL and DDFT.

Fig. 43-1 Intraoperative photograph of a Warmblood gelding with chronic, recurrent proximal suspensory desmitis of the left hindlimb after a surgical approach to sever the deep branch of the lateral plantar nerve (L Pl n). The horse is in right lateral recumbency (proximal is to the right and dorsal is uppermost), the Kelly forceps is positioned at the level of the tarsometatarsal joint (TMt Jt), and the Senn retractors are positioned deep to the metatarsal fascia and superficial digital flexor tendon, both of which are retracted in a plantar direction. The parent lateral plantar nerve (L Pl n) coursing distally originates from the tibial nerve proximal to the tarsus and divides into a deep branch, a segment of which is removed during neurectomy, and then continues distally as the lateral plantar nerve. The deep branch further divides within the suspensory ligament and proximal metatarsal region to innervate the proximal aspect of the suspensory ligament and the plantar cortex of the third metatarsal bone; it then divides into lateral and medial plantar metatarsal nerves, which continue distally to innervate the fetlock joint (not shown, deep to the suspensory ligament).

Conformation

Sickle-hock and more important straight hindlimb conformation predispose to metatarsal region lameness (see Chapter 4).

Clinical Characteristics and Diagnosis of Lameness in the Metatarsal Region

There are no pathognomonic historical findings related to metatarsal region pain causing lameness. Palpation of the metatarsal region should be done with the horse in standing and non–weight-bearing positions. The metatarsal region cannot be adequately palpated when the horse is standing, because soft tissue structures in the proximal metatarsal region are not readily palpable in this position. Careful palpation for signs of inflammation and bony swelling should be performed with the limb in both positions. Even when the limb is flexed, it is still difficult to define the SL and DDFT proximally. However, it is possible to apply pressure over the proximal aspect of the SL to check for a painful response. In some horses with PSD, edema may be present or the area may feel full or slightly thickened, but dense fascia covering the SL and DDFT prevents expansion, and swelling may go undetected. A painful response to deep palpation over the proximal aspect of the medial splint bone and distal aspect of the tarsus, referred to as the Churchill test, may indicate the presence of referred pain and primary distal hock joint lameness, but in my experience this is a nonspecific test. Many horses with hindlimb lameness originating from sites other than the distal hock joints manifest a positive response to compression of both the medial and lateral splint bones and distal, medial aspect of the tarsus. Palpation of the dorsal cortex of the MtIII may reveal pain in trotters or other horses that interfere and may be an important finding, but palpation may not necessarily localize the primary source of pain that is causing lameness. Careful palpation of the abaxial and axial aspects of both splint bones is necessary to uncover hidden splint exostoses. Even subtle enlargement of the SL body, as it emerges from the bony encasement of the splint bones, that is accompanied by a painful response can indicate early suspensory desmitis. Both branches of the SL and attachments to the PSBs should be carefully palpated. Horses with a history of curb may develop progressive SDF tendonitis in the proximal metatarsal region, but this can easily be overlooked unless the area is carefully palpated (see Chapters 6, 69, and 78).

Diagnostic Analgesia

The high plantar perineural block (see Chapter 10) should be used to localize pain to the metatarsal region. Medial and lateral plantar nerves and medial and lateral plantar metatarsal nerves are blocked just distal (approximately 1.5 cm) to the tarsometatarsal joint (see Chapter 10). Variations of this block are often used, but it is important to recognize that subtarsal analgesic techniques targeting the lateral plantar nerve, or its deep branch, will not block the medial plantar nerve, and false-negative results could be obtained. More important, false-positive results incriminating the proximal aspect of the SL as a source of pain can occur. Subtarsal analgesia should be performed only after results of low plantar analgesia are observed, because most injection techniques in the subtarsal region are likely to desensitize the plantar metatarsal nerves, important contributors to innervation of the metatarsophalangeal joint. An injection technique was recently described for the diagnosis of PSD; the lateral plantar nerve is blocked approximately 15 mm distal to the head of the MtIV, just axial to the MtIV, at a depth of 25 mm.⁷ Although in theory this block is done in close proximity to the deep branch of the lateral plantar nerve, it is within the same fascial compartment as the parent branch, leading to the possibility of blocking this important contributor to distal limb innervation. If completed as a stand-alone technique without first performing low plantar analgesia, subtarsal analgesia may lead the clinician to the erroneous impression that pain is emanating from the proximal aspect of the SL. Management of horses with PSD by desmoplasty and fasciotomy was recently described, but criteria for inclusion of cases suggested that pain was localized to the proximal metacarpal or metatarsal region by use of only subcarpal or subtarsal analgesia without first blocking the distal aspect of the limb.⁸

In the forelimb, distal palmar outpouchings of the carpometacarpal joint complicate interpretation of diagnostic analgesic techniques. In the hindlimb, distal plantar outpouchings of the tarsometatarsal joint can potentially be penetrated when high plantar analgesic techniques are performed, but outpouchings of the joint were not seen in magnetic resonance images of cadaver specimens in a study comparing imaging modalities in the plantar metatarsal region.⁹ In an in vivo radiographic contrast study in only 5% of limbs was the tarsometatarsal joint inadvertently penetrated when injection mimicking high plantar analgesia was performed at a level of 1.5 cm distal to the tarsometatarsal joint, although in the same study inadvertent penetration of the tarsal sheath occurred in 40% of limbs.¹⁰ Care must be taken when interpreting diagnostic analgesia.