Introduction

Cruciate disease is the most commonly operated orthopaedic disease in dogs and the main cause of stifle joint osteoarthritis (OA). Concurrent medial meniscal injury secondary to cruciate instability has been reported in 30-70% of cases^1-5. Cruciate instability causes cranial tibial translation with relative caudal subluxation of the femoral condyles. The medial femoral condyle moves caudally on flexion and cranially on extension of the stifle joint causing shear stress primarily on the caudal pole of the medial meniscus.

Lateral meniscal injuries are rare and the reported incidence ranges between 0-2% of all meniscal injuries.

Functional menisci improve congruency between the femoral condyles and the tibial plateau and provide critical roles in dissipation of compressive loads ('shock absorption'), stability, reduction in friction and protection of articular cartilage.

Loss of meniscal integrity impairs the ability of the meniscus to dissipate compressive forces transmitted through the stifle joint. This leads to supra-physiologic loads on the articular cartilage of the joint resulting in pain, lameness and more rapid progression of OA.

Failure to identify and treat meniscal injury at the time of cruciate surgery is a common cause of poor response to cruciate surgery. Untreated meniscal injuries are a major cause of prolonged morbidity and poor function following cruciate surgery in dogs.

It is important to differentiate between untreated meniscal injuries and 'late' meniscal injuries. Late meniscal injuries are defined as a meniscal injury that occurs subsequent to cruciate surgery where the meniscus was correctly determined to be normal or where a meniscal injury was appropriately treated. Late meniscal injuries are a recognised complication of all types of cruciate surgery with a reported incidence between 4-22% depending on the method of cruciate surgery and on the method of stifle joint inspection^3,6.

Meniscal release (surgical transection of the medial meniscus with the intention of reducing the likelihood of subsequent meniscal injury) has been shown to significantly impair pressure distribution by the medial meniscus with detrimental effects not significantly different to caudal pole hemimeniscectomy. Prophylactic meniscal release is not recommended unless there is a significant risk of late meniscal injury⁶.

Relevant Meniscal Anatomy

The menisci are biconcave, C-shaped fibrocartilaginous discs with their open part directed towards the axis of the bone. In cross section the menisci are wedge-shaped being thickest on their convex abaxial border. The menisci are held in position by six meniscal ligaments. Both menisci are attached to the tibia by a cranial and caudal meniscotibial ligament. An intermeniscal ligament joins the cranial horns of each meniscus and lies immediately cranial to the tibial insertion of the cranial cruciate ligament. This is an important landmark for placing a stifle distractor. The caudal horn of the lateral meniscus is also attached to the caudal part of the medial femoral condyle by the meniscofemoral ligament of the lateral meniscus. The medial meniscus lacks any femoral attachment.

There are five key points to know about meniscal anatomy:

1.  The medial and lateral menisci are different.

2.  The caudal meniscotibial ligament (medial meniscus): very commonly a part of this ligament needs to be transected to remove a bucket handle tear or to perform a caudal pole hemimeniscectomy. There is no capsular attachment at the area of the caudal meniscotibial ligament; a probe can pass freely dorsal and ventral to the ligament.

3.  The menisco-femoral ligament of the lateral meniscus: this is the largest of the meniscal ligaments and while normal is very different to the medial meniscus.

4.  The medial meniscus is firmly attached to the tibial plateau through peripheral attachments to the joint capsule and the medial collateral ligament. The lateral meniscus has no attachment to the lateral collateral ligament and has no caudal capsular attachments. Only the cranial third of the lateral meniscus has capsular attachments. It is this difference in attachment that renders the medial meniscus less mobile than the lateral meniscus and explains the high incidence of damage to the caudal pole of the medial meniscus. When using a meniscal probe to inspect the menisci it is important to recognise that the capsule should be firmly attached to the entire periphery of the medial meniscus.

5.  Normal menisci are gloss white. Damaged menisci typically have a matt appearance and are discoloured.

Treatment of Meniscal Injury

Surgery is the treatment of choice for meniscal injury. Conservative treatment is not recommended due to the avascular nature of the majority of the meniscus and consequent lack of healing. Only the peripheral 10%-15% of the meniscus has a blood supply. The remainder of the meniscus receives nutrition from the synovial fluid. Dogs with untreated meniscal injuries remain with significant lameness.

Types of Meniscal Injury

There are three common types of meniscal injury; bucket handle tears, peripheral capsular detachment and radial tears. All occur predominantly only in the caudal 1/3 of the medial meniscus between the medial collateral ligament and the caudal meniscotibial ligament.

Bucket Handle Tears

These are the most common meniscal injury. These arelongitudinal tears (parallel to the circular orientation of the collagen fibres) named as the inner part of the meniscus is axially displaced resembling the handle of a bucket. Probing with a meniscal probe may be necessary to 'unmask' a bucket handle tear. When probing it is important to remember that in the normal medial meniscus, passage of the probe dorsally and ventrally over the meniscus will not cause damage or separation and that (other than at the area of the caudal meniscotibial ligament) the probe cannot be passed caudally beyond the meniscus due to the close attachment of the joint capsule to the medial meniscus. (This is not the case with the lateral meniscus where the probe passes freely dorsally and ventrally). Multiple bucket handle tears can be present in the meniscus.

Peripheral Capsular Detachment

This is similar to a large bucket handle tear with complete detachment of the caudal pole of the meniscus from the joint capsule. These are uncommon.

Radial or Transverse Tears

These are full thickness tears radiating from the inner concave (axial) border and are uncommon.

Partial Meniscectomy

Partial meniscectomy (removal of only the damaged part of the meniscus) is strongly preferable to total meniscectomy.

Axial partial meniscectomy is the removal of the 'bucket handle' part of a bucket handle tear. This leaves the periphery of the meniscus intact and preserves some of the load bearing capacity of the meniscus. Caudal pole hemimeniscectomy is the removal of the entire caudal pole of a detached medial meniscus. While this is necessary in cases where the entire caudal horn is detached it inactivates the shock absorbing capacity of the meniscus and increases the severity of subsequent osteoarthritis similar to total meniscectomy⁷.

Equipment

Recommended equipment for stifle joint examination:

 2 Gelpi self-retaining retractors

 Stifle joint distractor (http://www.vetinst.com/) or narrow-bladed (12 mm) Hohmann retractor

 2 sharp-pointed Senn retractors

 mosquito haemostat or meniscal forceps

 Frazier suction tip and suction

 Meniscal probe (http://www.vetinst.com/)

To properly inspect the menisci when a cranial cruciate ligament rupture exists it is essential that cranial drawer of the tibia is achieved. Two methods exist in which this may be achieved. The first method is the use of a self-retaining stifle joint distractor. One of the distractor tips is placed in the proximal part of the intercondylar fossa. The second tip is positioned under the intermeniscal ligament (immediately cranial to the insertion of the cranial cruciate ligament). The second method involves a combination of a narrow-bladed Hohmann retractor and a sharp pointed Senn retractor and necessitates a surgical assistant to achieve exposure and free the surgeon to perform the meniscal inspection. The Senn retractor is placed into the infrapatellar fat pad and the tibia pulled cranially. The Hohmann retractor is inserted through the intercondylar space of the femur and hooked over the caudal aspect of the tibial plateau to lever the femur caudally.

Use of a meniscal probe greatly increase sensitivity in detecting meniscal injuries and is strongly recommended.

References

1.  Harasen G. Canine cranial cruciate ligament rupture in profile. Can Vet J 44:845-846, 2003

2.  Mahn MM, Cook JL, Cook CR, et al. Arthroscopic verification of ultrasonographic diagnosis of meniscal pathology in dogs. Vet Surg 34:318-323, 2005

3.  Pacchiana PD, Morris E, Gillings SL, et al. Surgical and postoperative complications associated with TPLO. J Am Vet Med Assoc 222:184-193, 2003

4.  Ralphs SC, Whitney WO. Arthroscopic evaluation of menisci in dogs with cranial cruciate ligament injuries: 100 cases. JAVMA 221:1601-1604, 2002

5.  Timmermann C, Meyer-Lindenberg A, Nolte I. Meniscus injuries in dogs with rupture of the cruciate ligament. Dtsch Tierarztl Wochenschr 105:374-377, 1998

6.  Lafaver S, Miller N, Stubbs W, et al. Tibial Tuberosity Advancement. Vet Surg 36:573-586, 2007

7.  Johnson KA, Francis DJ, Manley PA, et al. Comparison of the effects of caudal pole hemi-meniscectomy and complete medial meniscectomy in the canine stifle joint. Am J Vet Res 2004; 65:1053-1060.