Exploration of the Stifle Joint and Treatment of Meniscal Injuries

James Tomlinson, DVM, Diplomate American College of Veterinary Surgeons

 

The stifle joint is the most commonly operated on joint in the dog because of cranial cruciate ligament ruptures and patella luxations.  A good working knowledge of the stifle joint is useful so that abnormalities can be identified at surgery.

Surgical Exploration of the Stifle

One of the most important aspects of being able to inspect the stifle joint is to have adequate exposure of the joint.  It can be very frustrating to try to inspect the joint or repair the problem if adequate exposure is not present.  The stifle joint is most often approached through a standard lateral approach to the stifle joint.  Some surgeons prefer a medial approach to the joint, however.  The common approaches to the stifle joint are well described in Piermattei's An Atlas to the Surgical Approaches to the Bones of the Dog and Cat (WB Saunders Co., 3rd edition, pp 272-281).  The extent and placement of the incision varies slightly depending on the procedure being performed.  The joint is always opened to check for pathology within the joint.  One point of importance is to make sure that the joint capsule is incised all the way to the top of the joint.  This allows one to adequately luxate the patella medially.  To do this from the lateral approach, the vastus lateralis muscle must be dissected off of the top part of the joint capsule.  This will make incising of the top part of the joint capsule easy and also prevent one from cutting into the muscle.  Once the joint is exposed, a set routine of inspection of the joint should be followed every time before proceeding with any correction of a problem.  This hopefully will prevent missing any abnormality which may change the way the repair is handled.

Joint Capsule

Look for the color and thickness of the joint capsule after cutting through it.  The normal lining to the joint should be a light pinkish red to white color with a very smooth surface.  Thickness of the capsule will vary with the size of the dog.  A congested, reddish-gray, thickened lining may be indicative of an inflammatory lesion such as chronic arthritis, rheumatoid arthritis, SLE, or infection.  Biopsy of the joint capsule may be indicated and may provide valuable information.  If infection is suspected, the joint capsule is cultured in blood culture medium (soy tryptokase broth).

Joint Fluid

When the joint capsule is opened, the joint fluid is judged as to whether the amount of joint fluid is increased, normal, or decreased.  A normal joint, even in a large dog, will rarely have more than 1 ml of fluid.  The color and viscosity of the joint fluid should also be assessed.  The normal color of the fluid is a light straw yellow.  The joint fluid should string out 1-2 inches between two fingers if the viscosity is normal.  If the joint fluid is to be collected for culture or analysis, a sample is collected with a needle and syringe just before opening the joint capsule.  Joint fluid should be cultured in blood culture media if indicated.

Cartilage Surface

The normal articular surface should be a glistening white color with a smooth even surface.  In the older animal, the cartilage may take on a slightly yellowish tinge.  Look for discoloration, erosions, and osteophyte production of the cartilage.  Osteophytes are most commonly found along the medial and lateral side of the trochlear ridges, the proximal end of the trochlear groove, the intercondylar notch, and on the proximal and distal poles of the patella.  The shape and depth of the trochlear groove should be assessed.  The rule of thumb is that the trochlear groove should be at least 50% of the cranial-caudal thickness of the patella.  The under side of the patella should also be inspected for cartilage damage.  Especially with patellar luxations, erosion of the cartilage of the patella and medial trochlear ridges is common, and if present, decreases the prognosis for limb function.

Cruciate Ligaments

The cruciate ligaments should be situated between the two condyles of the femur.  The cranial cruciate ligament is the most cranial and is the ligament best seen unless it is ruptured.  The cranial cruciate ligament attaches to the medial side of the caudal aspect of the femoral condyle and runs cranial, medial and distal to attach to the intercondylar crest area of the proximal tibia just caudal to the intermeniscal ligament.  The caudal cruciate ligament attaches to the cranial aspect of the lateral side of the medial femoral condyle and runs caudally to attach to the lateral edge of the popliteal notch of the tibia.  Normally you will only see a portion of the cranial cruciate ligament and the proximal end of the caudal cruciate ligament.  The ligaments should be a pale white color and individual fibers of the ligament should be apparent.  A synovial layer should cover the ligaments and blood vessels can be seen on the surface of the ligaments.  The cranial cruciate ligament fibers will spiral about 90 degrees externally when the knee is flexed.  If the cranial cruciate ligament is ruptured, the caudal cruciate ligament is more visible.  Partial ruptures of the cranial cruciate ligament do occur and cause lameness.  Variable amounts of the ligament may be torn from just a small amount to more than 90%.  Classically, the cranial medial band of the ligament will tear first.  Caudal lateral part tears have been observed in some of the straight-legged dogs such as Rottweilers.  Partial tears of the cranial cruciate ligament occur commonly and canbe missed without careful inspection.  If rupture of the cranial cruciate ligament has occurred, the remnants of the ligament are removed with a #11 scalpel blade.  This will enhance visualization of the caudal cruciate ligament and both meniscus.  In young dogs, avulsion injuries of the cruciate ligament occur.  Avulsion of the femoral attachment of the caudal cruciate ligament is the most common injury of this type that is seen.

Tendons

The long digital extensor tendon attaches to the lateral side of the distal femur on the extensor fossa.  The tendon is intracapsular but is extrasynovial in that it is covered by synovium.  In young dogs, avulsion injuries of the long digital extensor tendon occur.  In dogs with lateral patellar luxations, the patella will abrade the tendon, which may lead to rupture of the tendon.  The popliteal tendon attaches just caudal to the long digital extensor tendon, but is just outside of the joint capsule.  To expose the attachment site of the popliteal tendon, a small incision is made through the joint capsule just caudal to the long digital extensor tendon. 

Collateral Ligaments

The medial and lateral collateral ligaments are not visible from within the joint.  Collateral ligament injury is suspected on the basis of preoperative instability by stressing the joint in varus or valgus manner.  To visualize the ligaments, an extracapsular approach directly over the ligament is made.  The lateral collateral ligament arises on the lateral epicondyle of the femur and inserts on the fibular head.  The medial collateral ligament originates on the medial epicondyle of the femur, and inserts directly distal by a short and long branch on the medial side of the tibia.

Meniscus

The stifle joints are the only joints of the body, except the temporomandibular joint, to have menisci.  The menisci are very important to the normal function of the stifle and are frequently injured in conjunction with cranial cruciate disease. The function of the meniscus has been described as (1) protecting the opposing articular surfaces by acting as shock absorbers, (2) increasing the stability of the joint by deepening the articular surface of the tibial plateau, (3) relieving the incongruity between the femur and the tibia by acting as peripheral, elastic, movable washers, and (4) aiding in lubrication of the joint.  In association with cruciate ruptures, a meniscal injury will be present about 30-70% of the time.

The menisci are biconcave C-shaped discs of fibrocartilage, which open toward the axis of the bone.  The cross section of the meniscus is wedge shaped, with the outer edge (abaxial) thicker and the inner edge (axial) thin.  The shape of the meniscus is adapted to the corresponding structures of the articular surface of the tibia and femur.  The lateral meniscus is slightly thicker, forms a slightly greater arc, and is more concave than the medial meniscus.  The menisci are held in place by six meniscal ligaments.  Both menisci have a cranial and caudal meniscotibial ligament.  The caudal horn of the lateral meniscus also has a femoromeniscal ligament, which runs from the caudal horn of the lateral meniscus to the lateral aspect of the medial femoral condyle.  An intermeniscal ligament joins the cranial poles of the two menisci and lies just cranial to the tibial insertion of the cranial cruciate ligament.

The medial meniscus also has a fibrous attachment to the medial collateral ligament and its periphery is attached to the joint capsule by coronary ligaments.  The lateral meniscus has no attachment to the lateral collateral ligament and loose attachments to the joint capsule.  The blood supply to the menisci arises from branches of the lateral and medial geniculate arteries.  Branches from these vessels supply the joint capsule that in turn provides vessels to the menisci. Only the outer 15-25% of the menisci has a blood supply.  The inner 75-85% of the menisci is devoid of vessels and relies on diffusion of nutrients from the joint fluid for nutrition.

The stifle is a complex hinge joint that has motion in two planes.  The menisci move during flexion, extension, and rotary motion of the stifle.  As the stifle is flexed, the menisci move caudally on the tibia plateau.  Because of the attachments to the medial collateral ligament and the joint capsule, the medial meniscus displaces considerably less than the lateral meniscus.  In extreme flexion, the lateral meniscus may protrude over the edge of the tibial plateau.

Conversely, as the stifle extends, both menisci slide cranial on the tibial plateau.  Injury to the menisci occurs when a meniscus is subjected to abnormal tension or pressure.  This usually occurs when the joint is undergoing combined flexion-rotation or extension-rotation.  The portion of the meniscus that is injured depends on the degree of flexion of the stifle when the rotational force is applied.  In extension, the cranial portion of the stifle is injured, whereas in flexion the caudal portion of the stifle is injured.  Direction of rotation determines which meniscus is injured.  The medial meniscus is typically damaged in internal rotation.

When the medial meniscus is injured, it is almost always in conjunction with rupture of the cranial cruciate ligament.  When the cranial cruciate ligament is ruptured there is an abnormal increase in internal rotation of the tibia, especially in flexion.  This increase in internal rotation allows the medial femoral condyle to move farther caudal than the relative immobile medial meniscus.  The effect is that the medial femoral condyle is centered over the caudal pole of the meniscus.  Excessive twisting forces on the caudal pole of the medial meniscus result in longitudinal injuries to the meniscus.  In extreme flexion, the medial meniscus is compressed between the tibia and femur and can cause tearing of the peripheral attachments of the meniscus.  This type of lesion frequently causes a clicking sound during range of motion of the joint.  Cranial displacement of a torn meniscus may occur during flexion and extension of the joint.  Decrease in cranial drawer motion may occur if the torn portion of the meniscus is displaced into the cranial aspect of the joint.

One of the most important aspects of being able to inspect the stifle joint is to have adequate exposure of the joint.  It can be very frustrating to try to inspect the joint or repair the problem if adequate exposure is not present.  The stifle joint is most often approached through a standard lateral approach to the stifle joint (Piermattei DL. An Atlas of Surgical Approaches to the Bones and Joints of the Dog and Cat. 3nd ed., Philadelphia: WB Saunders 1993:276-277).  Some surgeons prefer a medial approach to the joint, however. The common approaches to the stifle joint are well described in Piermattei's An Atlas to the Surgical Approaches to the Bones of the Dog and Cat.  The extent and placement of the incision varies slightly depending on the procedure being performed.  The joint is always opened to check for pathology within the joint. One point of importance is to make sure that the joint capsule is incised all the way to the top of the joint.  This allows one to adequately luxate the patella medially.  To do this from the lateral approach, the vastus lateralis muscle must be dissected off of the top part of the joint capsule.  This will make incising of the top part of the joint capsule easy and also prevent one from cutting into the muscle.  Once the joint is exposed, a set routine of inspection of the joint should be followed every time before proceeding with any correction of a problem.  This hopefully will prevent missing any abnormality that may change the way the repair is handled.

Two simple pieces of equipment will make inspection of the meniscus easy and are the trick to successful visualization of the meniscus.  The two pieces of equipment that are needed are the Senn retractor and the Hohmann retractor (baby).  The baby Hohmann retractors are available from Synthes, Ltd. (catalog #399.19 small - blade with 8 mm with short narrow tip).

The hooked end of the Senn retractor is hooked onto the fat pad at the front of the tibia and pulled cranial.  The Hohmann retractor is inserted between the condyles of the femur and hooked on the back of the tibia.  With the stifle flexed, the shaft of the Hohmann retractor is levered back against the trochlear groove of the femur.  This will allow a look into the back of the joint to see almost all of both menisci.  Any tears or folding over of the meniscus can be seen.  Removal of a damaged meniscus can then be done with the joint in this position.  The torn section of the meniscus typically will flip up to the cranial aspect of the joint making identification of a tear easier.  Three types of complete tears of the meniscus are recognized; transverse, longitudinal, and peripheral capsular detachments.  Incomplete tears of the meniscus occur and can be hard to detect.  Incomplete tears of the meniscus may become complete tears of the meniscus at a later date, and cause clinical signs even though the stifle has been stabilized.  A small arthroscopy probe can be used to detect tears of the tibial side of the meniscus.

When meniscal injury occurs, meniscectomy has been the traditional treatment of choice. Debate exists as to whether to perform partial or complete meniscectomy. Generally, the more meniscus  that is removed, the greater the degenerative changes that develop.  A #11 scalpel blade is typically used to remove the torn section of the meniscus. Care must be taken to avoid cutting the articular surface or the medial collateral ligament. A special cutting instrument called a pull blade is also useful for removing the torn section of the meniscus.  The pull blade is used in arthroscopic removal of the meniscus in people.

Regeneration of the meniscus has been reported following meniscectomy in both dogs and humans.  The regenerated tissue resembles fibrocartilage and appears to decrease degenerative joint disease.  Regeneration is not uniform and does not occur in every case. Menisci have been notorious for not healing due to minimal or no blood supply.  Experimentally, healing of a meniscal incision has been induced by creating vascular access channels and with fibrin glue.  In humans, suturing of peripheral meniscal tears can lead to healing of the lesion.  Special instrumentation has been developed to suture the meniscus arthroscopically.  Tissue anchors are used to anchor the suture material after the anchor has been inserted through the meniscus.  A special instrument has been developed to tie the suture material within the joint.  Meniscal allografts have also been performed with varying results.

In an attempt to prevent meniscal tearing subsequent to repair of a cranial cruciate rupture, meniscal release has been proposed.  The idea is to make the medial meniscus more mobile so that hopefully it can move with the medial condyle of the femur and thus prevent the meniscus from being subjected to abnormal stress.  No definitive documentation of the benefits of this concept has been published. Two methods of performing a meniscal release have been described.  The first method involves performing a radial (transverse) cut in the meniscus just caudal to the medial collateral ligament.  The second method involves transection of the caudal meniscotibial ligament of the medial meniscus.