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ОглавлениеCarpus
Dean W. Richardson
5.1 Description
Slab fractures of the cuboidal bones of the carpus are fairly common injuries in racehorses [1, 2] but very unusual in horses used for other activities. The third carpal bone (C3) is the affected bone in more than 90% of carpal slab fractures, although the radial, intermediate, ulnar, and fourth carpal bones can also be affected. The clinical signs are typically dramatic with obvious joint effusion, pain on carpal flexion, and moderate to severe lameness.
Rule: repair any lesion that is clearly visible radiographically on a lateral or DPLMO projection.
Although it is possible for C3 slab fractures to heal with rest alone, it is advisable to repair any lesion that is radiographically evident on a lateral or DLPMO projection [3]. Radiolucent lines that are seen solely on the tangential view may involve only the subchondral bone of the proximal joint surface [4] and therefore not require internal fixation. Prognostic considerations include comminution at the joint surface, marginal osteophytes, loose fragments in the palmar-lateral joint space (an indication of comminution), size of the fragment, and degree of displacement.
5.2 Surgical procedure
Use the arthroscope to more thoroughly evaluate the joint.
Do not position the scope too close to the distal row of carpal bones.
Arthroscopy during screw fixation enables a more thorough evaluation of the entire articulation [5]. It allows a visual check of accuracy of reduction/fixation while minimizing soft tissue trauma [6]. Arthroscopic technique follows basic principles [7]. The scope is positioned between the extensor carpi radialis and common digital extensor tendons when the fracture is in its typical location in the radial facet (Fig. F5A). If the fracture is in a more frontal plane and extends into the intermediate facet, it is important not to position the scope too close to the distal row of carpal bones. Otherwise, it may be difficult to see the intermediate facet clearly. If the fracture affects the dorsolateral corner of the intermediate facet, the scope is inserted medial to the extensor carpi redialis.
Fig. F5A: An overview of the relationships of the arthroscope to the extensor carpi radialis and the common digital extensor tendons, and of the needles used to mark the C3 fragment.
Displaced slab fractures: instrument portal on the opposite side of the joint.
In displaced slab fractures, an instrument portal is made on the opposite side of the joint. If possible, the instrument portal is made exactly at the margin of the fracture so that a curette can be inserted deeply into the fracture plane for debridement. It is essential to remove all loose fragments to allow accurate reduction.
Use needles for orientation during drilling.
After debridement (with a displaced fracture) or after examining the joint (with a non-displaced fracture), a 3" (7.5 cm)18 g spinal needle is placed in the joint just above the proximal edge of the center of the slab fragment. If the position is not central, a second needle is inserted in the correct position. Additional 1" (2.5 cm) 22 g needles are inserted at the medial and lateral margins of the fracture to verify the central positioning of the spinal needle. After the central needle is positioned correctly, a 22 g needle is inserted into the carpometacarpal joint immediately distal to the first needle (Fig. F5B, Fig. F5C). A #10 scalpel blade is used to make a deep incision reaching the face of C3 after measuring proximally from the carpometacarpal needle. It is usually possible to feel the dorsal ridge in the center of the face of C3 as the overlying soft tissue is incised.
Fig. F5B: The arthroscope is inserted into the midcarpal joint between the extensor carpi radialis and common digital extensor tendons, and directed medially. A spinal needle marks the intended screw direction, while 20 g needles mark the fracture site and the carpometacarpal joint.
Fig F5C: In the lateral view, the position of the needle in the carpometacarpal joint may be better appreciated.
Verify complete penetration of the fragment arthroscopically.
Use a K-wire as a guide for the centering insert.
The alignment of the spinal needle is checked arthroscopically. A 3.5 mm hole is drilled through the slab fragment using the spinal needle to guide the direction of the drill (Fig. F5D). A general alignment aid is to keep the bit perpendicular to the long axis of MC3. This assures that the drill remains parallel to the articular surface of C3 (Fig. F5E). With displaced fractures it is easy to check arthroscopically that the glide hole has reached the fracture, since the drill can actually be seen entering the fracture gap. With nondisplaced fractures, careful measurements and/or intraoperative radiographs are necessary. After removing the 3.5 mm bit, a 2 or 3 mm K-wire is placed into the hole through the drill guide. The guide is then removed and the centering insert is positioned by sliding it down the K-wire into the glide hole (Fig. F5F). With displaced fractures, the fragment can then be manipulated with the insert and a 3 mm K-wire to further ascertain the position and completeness of the glide hole. Do not attempt to manipulate the fragment with the drill bit since the bit may break.
Fig. F5D: The glide hole is prepared in the fracture fragment by drilling parallel to the previously placed spinal needle.
Fig. F5E: Other than the spinal needle, a good directional guide for drilling is the 90° relationship to the long axis of MC3. Maintaining this positioning insures that the joint surface will not be injured.
Fig. F5F: A K-wire placed in the glide hole serves as a marker over which the centering guide can be slid into position. This saves time and avoids undue soft tissue disturbance.
Flex the carpus prior to drilling the thread hole.
For single facet fractures, use 3.5 mm × 32 or 35 mm cortex screws.
The smaller head of the 3.5 mm cortex screw is advantageous.
The carpus is flexed, aiding reduction. Accuracy is checked again arthroscopically and the thread hole is drilled (Video DBASICS). Usually, it extends only about 40 mm, but palmar penetration does not constitute a problem. After making the countersink depression, the hole is measured and tapped routinely. For fractures that involve only a single facet, a 32 to 35 mm long 3.5 mm diameter cortex screw is adequate (Fig. X5A, Video 31018). If the fracture is large, i.e., involving both facets, two or sometimes three 3.5 mm screws are used (Fig. X5B). Alternatively, 4.5 mm screws can be used; the larger screw is preferred whenever there is marked comminution along the fracture plane or whenever stability is otherwise questioned. The 3.5 mm screw is greatly preferred for sagittal fractures of the radial facet [8] because its much smaller head allows it to be placed close to the C3-C2 articulation (Fig. X5C) without excessive countersinking [9]. In large slab fractures that involve both facets, the second screw inevitably passes through the extensor carpi radialis tendon and/or its sheath. The stab incision splitting the tendon‘s fibers should be made while holding the limb in flexion to achieve reduction, since subsequent manipulation will shift the relative position of the stab and the fragment.
Fig. X5A: Smaller fractures of the face of C3 involving only one articular facet may be repaired with one carefully placed screw.
Video DBASICS: Animation about drill basics.
Fig. X5B: For broad fractures of the face of C3, affecting both articular facets, two 3.5 mm cortex screws are employed.
Fig. X5C: The flatter head of the 3.5 mm screw makes it the implant of choice in areas with narrow tolerance as in this sagittal fracture of C3.
Video 31018: Slab fracture of C3.
Passive flexion is an essential component of postoperative physical therapy.
After the screw is inserted, the fracture line is probed and any remaining flaps are debrided. If there is a large fracture trough and a narrow remaining articular rim, the rim is removed with heavy rongeurs or a motorized burr.
For the surgeon not skilled in arthroscopy, repair via arthrotomy is a viable alternative.
Virtually every C3 slab can be repaired arthroscopically, although the advantages are questionable if the surgeon is not an experienced arthroscopist. An arthrotomy consists of a straight 5–6 cm incision located approximately 15 mm medial to the extensor carpi radialis tendon. The extensor carpi radialis tendon sheath should be avoided. A smooth tipped elevator can be placed in the incision and leverage used to retract the joint capsule. The fracture line is debrided with small angled curettes and/or bone picks. It is easiest to debride the fracture with the limb in partial flexion. Hard flexion closes the fracture line and tends to keep the fragment in reduction during placement of the screw. One or both screws must often be placed through a separate stab(s), since the desired position may not be directly under the arthrotomy incision. The standard lag screw technique is used as described above. An Esmarch bandage may be helpful and suction/ irrigation is indispensable.
Only skin sutures are used in the arthroscopic incisions, usually over the screw. If the screw incision is longer than 8–10 mm, a single subcutaneous synthetic absorbable suture is used.
5.3 Postoperative treatment
A lightly padded bandage is used for recovery and to help minimize swelling in the postoperative period. Passive flexion as a major component of physical therapy of the limb is strongly recommended to avoid loss of range of motion.
5.4 Complications
Three specific cautions bear mentioning:
Always keep a 2 or 3 mm diameter K-wire in the hole while changing bits, guides, and the tap. In this way, the hole will not be “lost” during its preparation. This is particularly important when placing a screw in the more central portion of C3 since the instruments and screw will be passing through the thickness of the extensor carpi radialis tendon and its sheath.
Because the thread hole does not usually pass through the palmar cortex, the hole has a “bottom”. Impacting the tap upon this unyielding barrier can result in stripped threads or a broken instrument. A small error in measurement and a screw that is slightly too long will result in failed compression since the screw head will not fully contact the fragment‘s cranial surface.
Note that the hexagonal socket of the 3.5 mm screw head is shallow and can be easily stripped if the screwdriver is not carefully seated.
5.5 Results
Most horses return to racing, albeit at a lower class.
The prognosis is not particularly good for a return to competition at former levels. Although the majority of horses will return to racing, most will drop in class [10]. The prognosis is better if the horse has raced previously, and even better if it has occasionally won. It is also improved by there being only minimal preexisting degenerative joint disease. In general the prognosis is better for Standardbreds than it is for Thoroughbreds.
Sufficient healing time is essential to successful treatment. Although horses with nondisplaced fractures sometimes resume work within 3–4 months, 8–10 months is a more common convalescent time frame. Radiographs are taken at 2–3 month intervals to assess the progress of healing.
5.6 References
1. Thrall DE, Lebel JL, O'Brien TR (1971) A five year study of the incidence and location of equine carpal bone fractures. J Am Vet Med Assoc; 159:1366.
2. Auer J (1980) Diseases of the carpus. Vet Clin North Am [Large Anim Pract]; 2:81–99.
3. Richardson DW (1990) Carpal bone fractures. In: White NA, Moore JN editors. Current Practice of Equine Surgery. Philadelphia: J.B. Lippincott Co., 566.
4. Ross MW, Richardson DW, Beroza GA (1989) Subchondral lucency of the third carpal bone in Standardbred racehorses: 13 cases (1982-1988). J Am Vet Med Assoc; 195:789.
5. McIlwraith CW (1992) Tearing of the medial palmar intercarpal ligament in the equine midcarpal joint. Equine Vet J; 24:367–371.
6. Wozasek GE, Moser KD (1991) Percutaneous screw fixation for fractures of the scaphoid [published erratum appears in (1991) J Bone Joint Surg [Br]; 73:524]. J Bone Joint Surg [Br]; 73:138–142.
7. Richardson DW (1986) Technique for arthroscopic repair of third carpal bone slab fractures in horses. J Am Vet Med Assoc; 188:288–291.
8. Fischer AT, Stover SM (1987) Sagittal fractures of the third carpal bone in horses: 12 cases. J Am Vet Med Assoc; 191:106.
9. Palmer SE (1983) Lag screw fixation of a sagittal fracture of the third carpal bone in a horse. Vet Surg; 12:54.
10. Stephens PR, Richardson DW, Spencer PA (1988) Slab fractures of the third carpal bone in Standardbreds and Thoroughbreds: 155 cases (1977–1984). J Am Vet Med Assoc; 193:353.
5.6.1 Online references
See online references on the PEOS internet home page for this chapter: http://www.aopublishing.org/PEOS/05.htm
