Histological features of meniscal injury

Vinesh Senan1, Jilsa Sucheendran2, Prasad3, Balagopal K4

 

 

1 Asst. Professor of Orthopaedics

2 Resident in Pathology

3 Addl. Professor of Pathology

4 Assoc. Professor of Orthopaedics

 

Govt. Medical College, Thrissur Kerala, India

 

 


Introduction

The menisci of the knee are part of a complex system designed to provide load sharing support and stability throughout the wide range of motion of the knee joint. In turn, the normal mechanical functioning of the meniscus is dependent on its proper biochemical composition, ultrastructural organisation and macroanatomical forms.

The human meniscus functions as a shock absorber for the normal knee. [10,48,62] It has been observed that meniscectomy results in osteoarthritis of the knee. [29,38] The interdependence of the ACL and medial meniscus on each other[50] have led surgeons to find new methods to repair torn meniscii. [4,5,41,59]

Injured tissues undergoes healing through the stages of inflammation, proliferation and remodelling, [3] which result in scar formation. However the meniscus fails to heal spontaneously[68]. Meniscus repair has shown variable rates of meniscus healing. Extraarticular tissues show a better healing rate with failures less than 2%.[39] Clinical failure rates may not estimate the healing rates in repaired meniscus thus meniscus may have failed to heal in as high as 45% of patients. [69]

Meniscus tissue being relatively acellular is susceptible to nonhealing of tears, [42,47,55,56] and it is thought that the presence of viable cells in the meniscus is important in the long-term survival of the tissue. [55] Previous investigators have reported patient age, [14,66] time since injury, [14,64,70] and instability [12,34,71] as risk factors for poor clinical outcome of meniscus repair. The cellularity of the meniscus will help the healing of the meniscus. The factors affecting the cellularity will increase the predictability of success of repair process. Correlation of cellularity with one of these risk factors could facilitate our understanding of the pathophysiology and predictability of meniscus repair failure.

In this study, our hypothesis was that patient age will significantly affect the healing rate of the meniscus. This will help the surgeon to predict the chances of meniscus healing preoperatively and to better understand the prognosis of the treatment.

Previous studies have shown that cellularity and healing of tissue have a direct corelation. [23,24,26,46,51].

 

Materials and methods

Thirty torn menisci were collected from patients aged 26 to 70 years at the time of arthroscopic meniscectomy. Ten cases following total knee replacement were used as controls. Seven lateral and twenty three medial torn menisci were included in the study. Five of the menisci were torn at the time of an ACL rupture. Patients with any other history of ipsilateral knee injury were excluded.

The torn menisci were removed arthroscopically using a punch and the meniscus was balanced to remove the entire torn section leaving a stable rim of meniscus tissue. Operative notes were used to document the appearance of the tear tissue as well as the status of the adjacent cartilage at time of partial meniscectomy.

 

Tear Classification

Torn menisci were classified into 3 major zones

Zone 1 - outer most third

Zone 2 - middle third

Zone 3 - inner most third

 

Histologic sample preparation

The menisci were fixed in formalin for 3 days. After fixation, specimens were embedded in paraffin and 5mm thick axial sections were cut and fixed on glass slides. Representative sections from each sample were stained with hematoxylin and eosin.

 

Histologic evaluation

Normal menisci contain two populations of cells: spindle shaped fibroblastic cells on the meniscal surface and rounder fibrochondrocytes in the interior. All menisci were evaluated for overall collagen organisation, presence of synovial layer, measures of cell number density and cell type at site of the meniscus tear. The mean cell number density was calculated counting the total number of cells present in twenty four 0.1-cm2fields arranged in a radial direction and dividing by the total area analysed.

 

Meniscus Histologic Scoring System

A total histologic score of 0 to 6 points was assigned according to the scale published by Rodeo et al. [56] For cellularity, the intrinsic cellularity was assigned points as shown in table 1.

 

Fibroblasts and fibrochondrocytes were identified when mean nuclear aspect ratio of intrinsic cells was 2.0 or more and less than 2.0 respectively.

 

Histomorphometry

The torn menisci were evaluated in a radial direction. Analysis in the radial direction was chosen as it was anticipated from previous studies in animals that synovial and vascular cell densities would vary as a function of distance from the peripheral rim. Similarly, in a previous study of intact menisci, 1 higher cell densities were identified near the anterior and posterior horns. All tears were evaluated radially at 1, 2, 4, and 6 mm from the inner edge. (Figure 1)

At each location, three 0.1-mm2areas were evaluated by determining the total cell number density, the nuclear morphologic characteristics, and the blood vessel density. Cellularity was determined by dividing the area of analysis by the total number of cells counted within that area. The nuclear morphologic characteristics were classified on the basis of the nuclear shape, with cells that had nuclei with a ratio of length divided by width of 1 classified as spheroid, between 1 and 2 classified as ovoid, and greater than 2 classified as fusiform as shown in previous studies

Statistical Analysis was done to determine the effect of patient age on the perimeniscal cellularity, intrinsic cellularity, intrinsic cell nuclear shape, and histologic score. [45] Descriptive statistics and x2 analysis were used to assess differences between groups. Within each zone, Student t tests were used to compare cell density between patients younger than 40 years and those 40 years and older.

 

Results

Quantitative Pathologic Characteristics:

Cell Density: 17 patients below the age of 40 years showed more cell density in the intact meniscus in zone3 compared to 13 patients >40 years( 423 113 vs 12176 P>.001). The cell number density in the zone 2 was also significantly greater in patients younger than 40 years than in patients older than 40 years (900 486 vs 522 243 cells/mm2, P = .013). In zone 1 the Intrinsic cellularity was significantly higher in younger patients than in those 40 years and older (1202 427 vs 746 305, P < .001).

 

Meniscal histologic score

Histologic score (Table 2) was related to tear site (P < .001), with worse scores found in inner zone.

The meniscus histology score was significantly lower in torn menisci retrieved at 36 weeks or more after injury than in the control menisci [P < .02). No significant correlation was found between patient age and histologic score [Spearman correlation coefficient = 0.01, P = .94).

Meniscus histology score [ranges from 0, which is the worst score, to 6, which is the best score) as a function of site of tear. (Figure 2)

The patient factors of age had significant effects on the histologic characteristics of the torn meniscus. Patients older than 40 years had a decreased intrinsic cellularity in the torn menisci compared with patients younger than 40 years. Decreasing histologic score of the meniscus tissue, had an increasing prevalence of Outerbridge II changes in the adjacent cartilage. Histologic score was related to tear site ( P < .001), with worse scores found in inner zone tear types compared with controls (both P < .01).

 

Discussion

Research in the past 20 years has further defined and clarified the multifaceted role of the meniscus. It has been shown to play an integral part in force transmission from the lower femur to the upper tibia [30,31,34].

These forces could be a partial explanation of the degenerative changes that occur in articular cartilage and subchondral bone after total meniscectomy. The meniscuss role in shock absorption and load transmission has also been clearly demonstrated in several biomechanical studies [13,14].

 

The effect of patient age:

Long term clinical outcomes have been poor in patients above 40 years[14,66] but short term results are good. 6 these patients have reportedly lower rates of meniscus healing and higher long-term failure rates of repair. One reaason for higher failure rates may be due to the decreased cellularity in older patients.

Earlier studies in animals have shown that a defect in the vascular zone heals successfully by migration of synovial cells over the surface of the meniscus. This establishes the connection between rates of healing and the cellularity of the meniscus. [9,25] Methods to enhance healing of tears in the avascular zone including implantation of synovium [11,20,31,36,61] and fibrin clot [5,46] have therefore focused on stimulation of the perimeniscal tissue growth toward the repair site. Although intrinsic cell proliferation has been noted in animal models of tears in the avascular zone during the first few weeks after injury, [46,47,72] no spontaneous closure of the untreated defects is observed in these tears, [5,20,24,26,65,73] even with suture repair. [30,52,61] These findings suggest that the intrinsic cells contained within the meniscus in the avascular zone may be incapable of mounting a sufficient repair response and that it is the perimeniscal growth that is more critical to successful healing. [5] In the current study, the factors of patient age and site of injury affects the cellularity (which leads to higher risk of degeneration and retear) and may contribute to increased failures of meniscus repair in older patients.

Our study only shows that age influences the way the meniscus responds to injury. Our study did not show the formation of any tissue bridging the tear ends. Further studies are required to study the effect of repair in the healing of meniscus and durability of these repair tissue to further injury, and wear and tear. The other problem of this study was that degenerated meniscus were used as controls.

 

Summary

The human meniscus shows a response to injury different from that seen in tissues that heal.

1. It does not show any proliferation of macrophages or neutrophils.

2. It does not show any scar tissue between the torn sites.

 

But it shows certain signs of healing like

a. Proliferation of fibroblasts and fibrochodrocytes

b. Hypertrophy and increased cellularity of the synovium

c. Modification of the cells to normal meniscal cells

d. Neovascularisation in the peripheral zone.

 

These findings suggest that the synovium plays an important role in the healing of meniscus and any repair process requires the synovium to be sutured to the tear to promote healing by bringing neovascularisation, neutrophils and macrophages to the site of injury. Further studies are required to study the effect of repair in the healing of meniscus. A deeper understanding of the biologic response of the human meniscus to injury is likely to not only help surgeons decide which tears may be at high risk for repair failure but may also lead to new approaches to facilitate regeneration and repair of this important musculoskeletal tissue.

 

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Correspondence should be sent to:

vineshsenan@yahoo.com

 

Cite this article as:

Vinesh Senan,,Jilsa Sucheendran, Prasad, Balagopal K. Histological features of meniscal injury. Kerala Journal of Orthopaedics. 2011;24:30:36

 

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