What is the clinical relevance of a tear of the knee meniscus? Has surgery any role in its treatment?

                        What is the clinical relevance of a tear of the knee meniscus?
                                       Has surgery any role in its treatment?     

                                                                 Dr KS DHILLON

 

Anatomy of the meniscus

Each knee joint has two wedge shaped semilunar fibrocartilages. The one between the lateral femoral and tibial condyles is called the lateral meniscus and the one between  the medial femoral and tibial condyle is called the medial meniscus. The meniscus consists of the body, anterior and posterior horns and the insertional ligaments. Anteriorly and posteriorly the meniscus is attached to the tibia plateau via the insertional ligaments. The menisci are approximately 35 mm in diameter and about 110 mm in length at its outer convex border where it is attached to the capsule (1). The anterior horns of both menisci are narrower as compared to posterior horn. The lateral meniscus covers about 80% of the lateral tibial plateau and the medial meniscus covers about 60% of the medial tibial plateau (1). 

The menisci provide articular congruency between semicircular shaped

femoral condyles and the relatively flat tibial plateau. The menisci significantly increase the contact area in the femorotibial articulation and hence they reduce the stresses

on tibial cartilage (1). The menisci bear about 40% to 70% of the load transmitted across the knee joint (2). This load bearing function is possible because of the strong anterior and posterior attachment of the menisci to the tibial plateau. This strong attachment prevents the menisci from dislodging from the joint when there is axial loading. 

A total meniscectomy will remove the stress distribution function of the meniscus leading to increased stresses on the tibial articular surface, which will subsequently lead to  osteochondral changes. In fact a radial transection through the meniscal body or a transection of insertional ligaments completely disables the load distribution function of

the meniscus and has the same functional effect as a total meniscectomy (1). The meniscus can still transmit significant loads when the central segment of the meniscus is removed provided that the peripheral circumferential fibres and insertional ligaments are intact (3).

Besides stress distribution, menisci are also believed to play significant roles in

shock absorption (4 and 5), joint stabilisation (6) and joint lubrication (7).

Day et al (8) studied the neurovascular supply of the meniscus. They found that the nerve supply follows the vascular pattern. The medial and lateral inferior and middle geniculate arteries supply blood to the perimeniscal plexus from which radial branches enter the meniscus at intervals. More blood flows to the anterior and posterior horns and less to the body of the meniscus. The body of the meniscus receives blood in its peripheral third only. Two thirds of the body is avascular along with an area close to popliteus tendon which is also avascular. Nerves are present in areas where there is blood flow to the meniscus.Most notably nerves are found anterior horn, posterior horn and the outer third of the body. However Zimny et al (9) showed the presence of neural elements in the middle third of the medial meniscus obtained at autopsy.There were no neural elements in the inner third.

Mine et al (10) studied the innervation of the medial and lateral menisci which were obtained from patients undergoing arthroplasty. They found that nerve fibre and sensory receptors were mainly found in the vascular outer third of the body and in the anterior and posterior horns. They found that nerve fibres positive for substance P were detected in the menisci but fewer in number. Their study showed that some pain in patients with meniscal tear could arise from the meniscus especial in peripheral tears which are associated with bleeding.

Three types of mechanoreceptors are found in the meniscus and these include Ruffini endings, Pacinian corpuscles, and Golgi tendon organs. They function as transducers which convert  physical stimuli of tension and compression into electrical nerve impulses (11).

Types of meniscal tears

Tears of the meniscus are broadly divided into 3 types: Basic, complex and displaced tears (12 and 13).

A.Basic tears

The basic tears are divided into 3 types:

1.Longitudinal tears

  Longitudinally oriented tears includes (a) horizontal tear and (b) longitudinal tear

A horizontal tear is also known as a cleavage tear and the tear is oriented horizontally parallel to the tibial plateau. It usually a degenerative tear which is seen in older individuals. It is commonly associated with a meniscal cyst. 

A longitudinal tear is also known as a vertical tear and it occurs perpendicular to the tibial plateau, along the long axis of the meniscus and parallel to the circumference of the meniscus. It most often involves the posterior horn of the meniscus. Such tears are often seen in young individuals following significant trauma.

 

2. Radial tears

 Radial tears extend radially from the free end of the meniscus to the periphery and are vertical to the tibial plateau. Such tears weaken the meniscus resulting in loss of function. They are commonly seen at the posterior horn of the medial meniscus and at the junction of the anterior horn and body of the lateral meniscus.

3.Meniscal root tears

Meniscal root tears (MRTs) are avulsion injuries or radial tears of the meniscal root insertion at tibial plateau. Such injuries are known to affect the function of the meniscus (14,15,16).

B.Complex tears

The complex tears of the meniscus are a combination of all or two of the following;  longitudinal,horizontal and radial-type tears.

 

C.Displaced tears

Displaced tears are those where the torn component is displaced but is either still attached to the parent meniscus or detached. There are 3 types of displaced tears:

Flap tears; where a horizontal or longitudinal tear is displaced.

Bucket-handle tear’ where a larger segment of a longitudinal tear is displaced.

Parrot beak tear is a displaced radial tear.

 

Etiology of meniscus tears

Tears of meniscus can be of three types, namely, traumatic, degenerative and iatrogenic. Traumatic tears usually result from sports related activities especially in young males. Degenerative tears are seen in older individuals after the age of 55 years. Iatrogenic injuries to the anterior meniscal roots occur during tibial tunnel reaming during anterior cruciate ligament reconstruction (17,18).

 

Traumatic tears result from a rotational varus or valgus force placed on a flexed knee. The medial meniscus is usually injured when the femur internally rotates and a valgus force is applied to the when the foot is fixed on the ground. A lateral meniscus tear usually results from a varus force on a flexed knee and the the femur rotates externally.

 

Degenerative tears are common and are due to degeneration of the meniscus which occurs with aging in older individuals. The tears are usually horizontal, oblique or complex (19).Bhattacharyya et al (20) carried out a Level I-1 study to assess the prevalence and clinical relevance of meniscal tears in individuals with OA of the knee using  MRIs of the knee. They found that in asymptomatic subjects with a mean age of 65 years, a tear was found in 76% of the  subjects, whereas in patients with symptomatic knee OA, a meniscal tear was found in 91% of the individuals.They concluded that meniscal tears are very common in both symptomatic and asymptomatic older individuals with OA of the knee. They also found that meniscal tears do not produce more pain in osteoarthritic knees as compared to osteoarthritic  knees with no tear of the meniscus. The meniscal tears in osteoarthritic knees do not affect the functional status of the individuals. The authors also that ‘these data do not support the routine use of magnetic resonance imaging for the evaluation and management of meniscal tears in patients with osteoarthritis of the knee’. 

It had been estimated in 2006  that some 30 million individuals in the USA over the age of 65 would have a meniscal tear (21)

Englund et al (22) did a population based study to assess the prevalence of meniscal tears in the general population and also to see the association of these tears with knee symptoms and with radiographic evidence of osteoarthritis.They found that the prevalence of a meniscal damage in the knee ranged from 19% in women between 50 to 59 years of age to 56% in men aged between 70 to 90 years of age.The overall prevalence of meniscal tears in the population was 35%. The prevalence of meniscus destruction was 10%. 

In individuals with evidence of OA the prevalence of tears was 63% in those who had knee symptoms ( knee pain, aching, or stiffness on most days) and 60% in those who had no symptoms. In individuals with no OA the prevalence of tears was 32% (with symptoms) and 23% (no symptoms) respectively.  Sixty-one percent of individuals with meniscal tears had no symptoms in the previous month.

They concluded that incidental findings of meniscal tears on MRI of the knee in the general population are common and that the incidence increases with age.

The etiology of degenerative meniscal tears of the knee is not known. Englund et al (23) in another study showed that meniscal tears in the knee are part of generalised osteoarthritic process. They did a study to see if radiographic hand OA is associated with meniscal damage in the knee. They found that the prevalence of meniscus damage in the knee increased with the number of fingers with OA.When 3 or more fingers had OA, the incidence of meniscal damage was 47.2%.Absence of OA in the knee did alter the prevalence rates. The authors concluded that the results suggest ‘a common non-age related etiologic pathway for both radiographic hand OA and meniscus damage’.

Treatment of meniscus tears

The treatment of meniscal injuries is controversial, partly because we know little about symptoms produced by meniscal tears. We know that many individuals with meniscal damage have no symptoms (20). We know that bucket handle tears of the meniscus do produce definite symptoms which are characteristic of such a tear. They produce locking of the knee, pain and sometimes giving way of the knee. Only in about 38% of patients with bucket handle tears, the tear is caused by definite trauma, 40% of such tears are caused by simple twist of the knee and in 20% there is no history of knee injury (24).  This probably means that significant numbers bucket handle tears are due to degeneration of the meniscus.The treatment of mechanical symptoms in patients with meniscal tears is controversial. However in certain individuals who present with locked knee after trauma the treatment is usually surgical.

The treatment of all other meniscal tears remains controversial. More and more data is becoming available which shows that there is no role of partial meniscectomy in the treatment of meniscal tears which are not producing mechanical symptoms. 

Sihvonen et al (25) carried out an excellent multicenter, randomized, double-blind, sham-controlled trial in 146 patients between the ages of 35 to 65 years who had no osteoarthritis of the knee but had knee symptoms consistent with a degenerative medial meniscus tear. The patients were randomly assigned to sham surgery or arthroscopic partial meniscectomy. The Lysholm score, WOMET score and score for knee pain after exercise were similar in the two groups. There was no significant difference between the two groups as far as rates for subsequent knee surgery were concerned.

Sihvonen et al (26) published a 2 years follow up of the patients in the study they published earlier in 2013 (25). The results were similar to the once in the earlier study. They found that the results of arthroscopic partial meniscectomy (APM) were not better than that of placebo surgery at 2 years follow up in patients with a degenerative meniscal tear and no knee osteoarthritis. They concluded that the results ‘support the evolving consensus that degenerative meniscus tear represents an (early) sign of knee osteoarthritis, rather than a clinical entity on its own’. They were of the opinion that patients with knee pain and suspected degenerative meniscal tears need not be sent for an MRI or APM even after failure of conservative treatment.

Arthroscopic partial meniscectomy is often used to treat mechanical symptoms due to meniscal tears. Sihvonen et al (27) in another study showed that resection of a torn meniscus by surgery has no added benefit over sham surgery in relieving mechanical symptoms such as knee catching or occasional locking. The mechanical symptoms may not likely be caused by the tear but may be due to the underlying degenerative joint disease.

Kise et al (28) carried out a randomised controlled superiority trial to determine if exercise therapy was superior to arthroscopic partial meniscectomy for knee function in

middle aged patients with degenerative meniscal tears. The study included 140 adults with a mean age of 49.5 years (range 35.7-59.9) who had degenerative medial meniscal tear which was verified by magnetic resonance imaging. Ninety-six percent of the patients had no definitive radiographic evidence of osteoarthritis. They divided the patients into two group. One group had 12 weeks of supervised physical therapy alone and the other group had arthroscopic partial meniscectomy alone. The patients were followed up for 2 years. They found no clinically relevant difference in the KOOS score  between the two groups at two years.

Khan et al (29) did a systematic review and meta-analysis to evaluate the efficacy of arthroscopic meniscal débridement as compared to non-operative or sham operation in patients who had knee pain and had mild or no  osteoarthritis of the knee. They found that there is moderate evidence to suggest that arthroscopic meniscus debridement for degenerative meniscal tears is not superior to nonoperative or sham treatments in middle-aged patients with mild or no concomitant osteoarthritis. 

There is general belief that patients with traumatic meniscal tears do better than patients with degenerative tears after partial meniscectomy. Thorlund et al (30) did a comparative prospective cohort study of 397 adults with traumatic or degenerative tears of the meniscus who underwent arthroscopic partial meniscectomy. The age range from 18 to 55 years with a mean age of 38.7 years. Forty two percent of the subjects were females. They were followed up for 52 weeks and at which time 14% of the patients were lost to follow up.

They found that the group with degenerative tears had better functional outcome as measured with KOOS scores compared to the traumatic tear group.However, there was no meaningful clinical difference between the groups at any time in the study. This study debunked long held belief that ‘patients with traumatic meniscal tears experience greater improvements in patient reported outcomes after arthroscopic partial meniscectomy than patients with degenerative tears’ (30).

Since, now that there is level I, sound, irrefutable, robust  evidence that arthroscopic partial meniscectomy is of no benefit to patients with degenerative meniscal tears, such surgery should not be recommended to patients with degenerative tears. 

Should partial meniscectomy be carried out in patients with traumatic tears which are not producing mechanical symptoms such as locking of the knee? Since evidence shows that the outcome after partial meniscectomy for degenerative and traumatic tears is the same, then most likely partial meniscectomy for traumatic tears will also be of no therapeutic value to patients.

Meniscal repairs are often recommended because of the believe that loss of meniscal function leads to osteoarthritis of the knee.Sommerlath and Gillquist (31) compared the functional results of meniscus repair and total meniscectomy in patients with meniscal tears in the vascular zone, with or without intact anterior cruciate ligament. They found that at 7-year follow-up, the functional outcome was similar in patient who had total meniscectomy or a repair of the meniscus.

 Rockborn and  Messner (32) studied the long term outcome of meniscectomy versus a meniscal repair in patients with meniscal tears.At the 13-year follow-up they found no difference between the groups in knee function, subjective complaints, or manual findings. Ninety percent of the patients in both groups had no knee problems during daily activities. They also found that at 13 years follow up the incidence and severity  of OA did not significantly differ in the two groups even when only the successful repairs were compared to meniscectomy.

 

Conclusion

The function of the knee meniscus is to distribute stress, absorb shock, provide joint stability and promote joint lubrication. Forty to seventy percent of the load that passess through the knee is borne by the meniscus. When the function of the meniscus is lost due to damage or excision of the meniscus, loads across the joint lead to osteochondral damage.

The outer third of the meniscus has neurovascular supply. Tears in the outer third can heal due to the vascularity and it forms the basis for meniscal repair. Some nerve endings have been found in the middle third. The nerve supply provides mechanoreceptor and some nocioreceptor function. 

Tears of the menisci occur due to either trauma or due to degeneration of the menisci which is part of the degenerative process of the knee joint. Degenerative tears can be symptomatic or asymptomatic. A significant portion of the population have asymptomatic tears of the menisci. In asymptomatic individuals with mean age of 65 years who has OA of the knee, the incidence of meniscal tears is about 76% and 91% in individuals with symptomatic knee OA. The incidence pain is not more in patients with knee OA who have a torn meniscus as compared to those with no tear. The presence of a torn meniscus in patients with knee OA is of no significance. 

In the past the treatment of meniscal tears have been surgical, in the form of arthroscopic meniscectomy or a meniscal repair. However, now there is very robust level I evidence that arthroscopic partial meniscectomy has no role in the treatment of degenerative tears of the meniscus even when there are mechanical symptoms. The outcome of partial meniscectomy in traumatic and degenerative meniscal tears is the same. Hence it is unlikely that patients with traumatic meniscal tears will benefit from a partial meniscectomy. The only exception maybe a patient who presents with a locked knee after trauma.

The long term outcome of meniscal repairs and meniscectomy is the same in terms of function and onset and severity of osteoarthritis. There appears to be no role of a meniscal repair in the treatment of meniscal tears. 

There appears to no basis for a recommendation of surgery for the treatment of meniscal tears of the knee except for in patients with post-traumatic locked knees.

 

 

 

 

 

 

 

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