Brian S. Beale, DVM, DACVS
Cranial cruciate ligament tears are common in small-breed dogs. Small-breed dogs have traditionally been treated successfully using an extracapsular stabilization technique, but in some cases this technique fails. One common reason for failure is excessive stress placed on the prosthetic ligament due to the presence of a steep tibial plateau angle. This lecture will discuss treatment options and decision-making for small-breed dogs having cranial cruciate ligament injury.
The extracapsular prosthetic ligament technique (EPLT) is the most common method of treating cranial cruciate ligament (CrCL) injuries in dogs and cats. While tibial plateau leveling osteotomy has become very popular for treatment of large dogs, EPLT remains a common method of treatment due to its lower cost and financial concerns of owners. The prosthetic ligament must be placed as isometrically as possible. Isometric positioning maintains similar tension on the ligament throughout the range of motion of the stifle, decreasing the chance of stretching or breaking the ligament, and allows more normal stifle movement.
A recent study by Hulse et al. assessed isometric positioning of a lateral extracapsular suture anchored to the lateral femoral condyle and the proximal tibia. The study evaluated 2 sites on the femur (F1, F2) and 3 sites on the tibia (T1, T2, T3). The most isometric position for a single lateral prosthetic extracapsular ligament is from the F2 site to the T3 site. If two prosthetic ligaments are to be placed, it is recommended that one ligament run from F2 to F3 and the other from F1 to T3. Proximal attachment is done around the femoropatellar ligament (F1 site) or near the origin of the lateral collateral ligament on the lateral femoral condyle (F2 site). The F2 site is located at the caudal extent of the condyle and at the same level as the distal pole of the lateral fabella of the gastrocnemius muscle. A bone anchor should be used when anchoring the ligament to the F2 site. A bone anchor can also be used at the F1 site or the suture may be anchored around the femorofabellar ligament. Distal attachment occurs at the proximal tibia using a bone tunnel, just caudal to the long digital extensor tendon. The hole for attachment should be positioned as proximal as possible in the tibia. Care should be taken to avoid drilling the hole too distal in the tibia because this location is not isometric. The prosthetic ligament can be attached on the femoral side directly to the femur with a bone anchor or around the femorofabellar ligament. When securing the suture to the femorofabellar ligament it is important to direct the suture around the bulk of the ligament rather than around the actual fabella. The ligament is a broad tight ligament that runs form the cranial aspect of the lateral fabella to the caudal aspect of the lateral femoral condyle. The most common mistake is for the surgeon to pass the prosthetic ligament around the fabella and not the femorofabellar ligament.
Bone anchors provide a quick, reliable and cost-effective method of securing the suture prosthesis to bone. Many different styles of veterinary bone anchors are available. The advantage of using bone anchors is that they can be placed with minimal invasiveness and can be placed at the recommended isometric sites. Bone anchors have excellent pull out strength and are designed to reduce abrasion on the prosthetic ligament. Presently, I recommend securing the prosthetic suture to a bone anchor on one end of the ligament only. The opposite end should be attached to a bone tunnel. Bone anchors are simple to place and have little instrumentation.
A bone tunnel can be used to attach the prosthetic ligament to the tibia. One or two holes are drilled through the proximal tibia at isometric locations and the suture prosthesis is passed through the hole or holes. If one hole is used, the suture is anchored to the medial aspect of the tibia using a suture button before passing the suture back through the bone tunnel and tying it to the complimentary end. If two holes are used, the suture is passed lateral to medial through the first hole, then medial to lateral through the second hole before tying to the complimentary end.
Excessive tension should not be placed on the suture when placing the knot. Excessive tensioning of the suture will increase the chance of suture failure and reduce the range of motion of the stifle due to over-constraint. The suture should be tensioned until cranial drawer motion is just eliminated. It is better to leave 1 or 2 mm of cranial drawer rather than over-constrain the joint.
Recently, a new ECLT technique has been described by Hulse et al. has been described for use in medium and large size dogs. The SwiveLock technique uses an extracapsular prosthetic ligament composed of 4 strands of fibertape from the F2 to T3 isometric sites. The ligament is placed through a tibial bone tunnel and is secured to the femur with a knotless suture button.
Tibial Plateau Leveling Osteotomy (TPLO)
Tibial Plateau Leveling Osteotomy (TPLO) is frequently performed to treat the cruciate-deficient stifle in medium and large sized dogs. Extracapsular repair techniques are most commonly used to treat small dogs with cranial cruciate deficiency. Extracapsular repair recreates the passive constraint of the stifle joint, while TPLO provides an active constraint of the stifle joint. Some small dogs have a marked slope of the tibial plateau, often exceeding 35°. Cranial tibial thrust increases as the slope of the tibial plateau increases. Greater cranial tibial thrust force places more strain on the extracapsular prosthetic ligament, increasing the chance for failure. TPLO has been proposed as a means of treating the small dog with cranial cruciate ligament deficiency, particularly when the tibial plateau angle is steep.
Indications for TPLO in Small Dogs
Potential indications for TPLO in small dogs are a steep TPA (e.g., greater than 30°, partial cranial cruciate ligament tears, failed extracapsular repairs and athletic small dogs. There has been no conclusive evidence that TPLO provides a better outcome that extracapsular repair at the present time.
TPLO technique used in small dogs is similar to large dogs. As would be expected, the instrumentation and implants for TPLO are proportionately smaller for smaller dogs. Saw blades are available for small dogs with a radius of 12, 15 or 18 mm. A smaller jig is available. Some surgeons prefer to omit the use of the jig when performing TPLO in small dogs. A 2.0 mm TPLO plate using 2.0 mm cortical screws is available for the smallest dogs weighing 4–10 kg. This plate must be contoured to match the contour of the tibia. A 2.7 mm plate TPLO is available for use in small dogs in the approximate weight range of 10–20 kg. The 2.7 mm plate is available as a pre-contoured plate and can accommodate locking or traditional cortical 2.7 mm screws. Exact contouring of the plate is not needed when using locking screws in the proximal tibia. Plate selection is sometimes modified from the above guidelines based if the width of the plate is inappropriate for the diameter of the tibia.
Practical tips when performing a TPLO in a small dog include:
1. The distal end of the plate may need to be tipped cranially when positioning the plate on the tibia so that good bone purchase can be achieved in all of the plate holes. The distal end of the plate can also be bent slightly caudally to achieve better purchase of the distal screw holes.
2. Contouring of the plate is sometimes more difficult due to the size of the plate, but accurate contouring is very important if not using locking screws. Improper plate contour leads to angulation or rotation of the limb.
3. The saw blade diameter selected should allow adequate width to the tibial tuberosity to prevent fracture during the postoperative period. If the tibial tuberosity segment is too narrow, the risk for avulsion fracture increases due to the pull of the patellar tendon.
4. A 0.035" or 0.045" k-wire is used for the temporary stabilization pin for most small dogs. It is important to place the pin as high as possible in the tibial tuberosity and lavage the pin during insertion to avoid thermal necrosis to reduce the chance of iatrogenic fracture. It is technically demanding to repair a fracture of the tibial tuberosity in a small dog after a TPLO is performed.
5. The small TPLO saw blades have a tendency to drift when starting the osteotomy, resulting in a large kerf. Drifting of the blade during cutting can be reduced by creation of a small cut in the bone at the cranial and caudal edge of the tibia to anchor the blade in place. Tip the saw slightly towards the axial plane of the tibia until the teeth of the blade engage, then slowly tip the saw back up to the desired 90° angle to complete the osteotomy.
6. The proximal screws must be placed very carefully when using the 2.0 mm TPLO plate because contouring the plate aligns the proximal holes in the direction of the joint surface. The proximal screws should be angled to avoid the joint surface. The fibular head is a good landmark that can be easily palpated to help place screws in an acceptable direction.
7. A medial meniscal release incision can be made if desired; however, the occurrence of postliminary meniscal tears may be less in small dogs.
8. Some small dogs will continue to have cranial tibial thrust after adjusting the TPA to 5°. Tibial thrust should be checked following placement of the temporary stabilizing pin, prior to placement of the TPLO plate. If tibial thrust persists, it is recommended that additional rotation of the proximal tibia be performed (e.g., over-rotation) or alternatively an extracapsular prosthetic ligament be placed to provide adjunctive stability.
9. TPLO can be performed in small dogs with an arthroscopically assisted mini-arthrotomy to minimize morbidity, enhance the view of partial tears of the cranial cruciate ligament and facilitate diagnosis and treatment of meniscal tears.
Small dogs typically do very well following TPLO. Most small dogs will begin to bear weight on the operated leg at a walk within a few days. Patients typically have mild weightbearing lameness at the time of suture removal. Healing of the osteotomy is expected to be adequate for normal activity at eight weeks postoperatively. Most patients reach maximal performance by 12 weeks postoperatively. Patients typically return to near normal function and are willing to perform daily tasks that were performed prior to their injury. Patients are expected to have minimal long term osteoarthritis. Postliminary meniscal tears are uncommon. Owner satisfaction is high.
The incidence of complications following TPLO in small dogs is less than that reported for medium and large dogs. The type of complications seen in small dogs include seroma, infection, screw loosening, tibial tuberosity fracture, fibular head fracture and over-rotation. It is unknown why incisional complications including infection and seroma occur less frequently in small dog TPLO patients. Most complications are minor and easily resolved. The need for revision surgery and plate removal is rare.