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SECTION II: ORIGINAL ARTICLES: Sports

Benefit of Single-leaf Resection for Horizontal Meniscus Tear

Haemer, Joseph M MS; Wang, Mark J MD; Carter, Dennis R PhD; Giori, Nicholas J MD, PhD

Author Information
Clinical Orthopaedics and Related Research: April 2007 - Volume 457 - Issue - p 194-202
doi: 10.1097/BLO.0b013e3180303b5c
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Abstract

Since Fairbank's initial observations in 1948 of changes in the knee after meniscectomy,12 subsequent researchers have tried to elucidate the function of the meniscus during weightbearing.5,23,30,33 Menisci perform a crucial role in the distribution of stress in the knee.1,24,39 Biomechanical studies have shown decreased contact areas and increased contact stresses after a total meniscectomy.1,5,21-33 Long-term followup of patients with previous meniscectomy suggests removing the meniscus represents a substantial risk factor for development of tibiofemoral osteoarthritis (OA).10,11,35 The likelihood of degenerative change in the knee seems related to the amount of meniscus resected.3,7,9,11,19 Some of the early biomechanical studies on meniscectomy centered on the effect of total meniscectomy.1,2,23 As surgical management evolved, the focus of biomechanical research shifted to the effects of partial meniscectomy on the knee.5,22,32 Findings of lower contact stresses in the knee after segmental versus total meniscectomy provide a biomechanical explanation for the lower rates of radiographic OA seen in long-term followup studies.5,22

Horizontal meniscus tears are seen frequently. In one study of 70 meniscal tears, horizontal meniscus tears were the most common type.13 In another study of 497 meniscus tears, 10% were classified as horizontal tears, and 51% as complex tears with a horizontal component.8 They often occur in the fourth and fifth decades of life, and can be associated with degenerative disorders of the meniscus.20,38 Metcalf and Barrett reported 59% of 1485 meniscus tears involved the posterior third of either meniscus, 27% involved the middle third and posterior third, and 0.5% involved all three segments.29 During arthroscopic débridement of a horizontal meniscus tear, a decision must be made to resect the superior or inferior leaflet, or both.

Although retaining meniscal tissue after a horizontal meniscus tear could have biomechanical advantages, retention must be balanced against the risk of retearing and reoperation. Patients with horizontal meniscus tears who have a partial meniscectomy have a higher risk (odds ratio, 5.5) of retearing and reoperation than patients with flap tears.34 Inadequate initial resection can cause retearing.36 Therefore it would be difficult to justify retaining one horizontal meniscus tear leaf if it offers minimal biomechanical advantages compared with resection of both leaves.

We asked whether sparing one leaf of a horizontal meniscus tear offered a biomechanical benefit over resection of both leaves, as indicated by increased contact area and decreased contact pressure. We wondered if the benefit of sparing one leaf depended on whether the tear involves the entire meniscus or is isolated to the posterior third, and to what extent the load-distributing function of the meniscus is preserved after resection compared with the intact meniscus.

MATERIALS AND METHODS

We used a mixed model, where measurements were made for each knee at each of four meniscus conditions: intact, horizontal tear (created with a scalpel), single-leaf resection, and double-leaf resection. Sheep have been studied extensively as an animal model of meniscectomy-induced OA.16,27,28,31 Because a supply of samples from animals of similar age and weight are easy to obtain, we used cadaveric sheep knees. The primary outcomes for the study were contact area, mean contact pressure, and peak contact pressure on the lateral tibial plateau of cadaveric sheep knees, measured at a load approximating that during gait. To capture measurement error, tests were repeated three times at each meniscus condition for each knee. To study the effect of tear size, this series of tests was performed for two cohorts, each of eight knees. In the first cohort, the horizontal tear was restricted to the posterior third of the meniscus, representing the most common clinical case. In the second cohort, the horizontal tear spanned the entire width and breadth of the meniscus, representing the limiting case.

We performed a power analysis to determine the least significant contrasts (LSCs) in contact area, mean pressure, and peak pressure that would be detectable with 80% power with significance of α = 0.05. Pilot data obtained with the second cohort (Table 1) were used to perform the power analysis using the methods suggested by Lenth.26 Using initial estimates of standard deviation, a mixed model ANOVA with eight samples and three repeats at each condition has LSCs of 12%, 9%, and 10% of the intact mean for contact area, mean contact pressure, and peak contact pressure, respectively (Table 1). In similar studies, Baratz et al5 reported decreases of 77% in contact area and increases of 146% in peak contact stresses with a 30% segmental meniscectomy (as for a bucket handle tear). Using measurement techniques similar to ours, Huang et al21 reported 65% increases in peak pressure, 90% increases in mean pressure, and 30% decreases in contact area with total meniscectomy. With LSCs less than the differences that were observed in similar studies, for other conditions correlated to negative OA outcomes, we concluded our study design with eight samples and three measurement repetitions would detect differences in contact area and pressure that would be relevant for long-term OA outcomes.

T1-30
TABLE 1:
Power Analysis for Least Significant Contrasts

Test samples consisted of 16 fresh-frozen knees from sheep 9 months to 1 year old with a live weight of 489 to 534 N. To facilitate repeatable fixturing, knees were aligned and trimmed, and potted: fluoroscopy was used to identify the joint line. Knees were flexed to 45°, which is the approximate angle of the knee during the stance phase of gait.37 The femur and tibia were trimmed parallel to the joint line 38 mm proximal and 44 mm distal to the joint line. The patella and patellar tendon then were removed. All soft tissue was removed 10 mm from the proximal and distal ends to facilitate adhesion of potting material. The femur and tibia were potted in polymethylmethacrylate (PMMA). Just before testing, the soft tissue surrounding the joint, except the joint capsule, collateral and cruciate ligaments, and menisci were removed to give access to the lateral meniscus (Fig 1).

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Fig 1:
A photograph of the anterior of the sheep knee sample shows the trimmed bone ends fixed in bone cement, parallel to the joint line. A packet with pressure-sensitive film is inserted between the lateral meniscus and lateral tibial plateau without disarticulating the knee.

The loading fixture was designed to allow compression and varus/valgus realignment while constraining flexion/extension and internal/external rotation (Figs 2 and 3). Freedom of varus/valgus realignment was allowed because a previous study showed it was necessary to accommodate the realignment of the joint in this plane after meniscectomy.30 Otherwise, the remaining intact meniscus would act as a spacer in the joint. The realignment was accommodated by loading through a spherical bearing which could reposition relative to the femur. Rotation and flexion were constrained to enable consistent load application between test repetitions and to stabilize the knee during loading. The fixture's vertical guides, anterior and posterior to the knee, provided the constraints on flexion and rotation. Load was applied with a servohydraulic test system (MTS 858 Mini Bionix, Eden Prairie, MN) and sensed via a 5-kN load cell (MTS) fixed beneath the bottom plate.

F2-30
Fig 2:
An illustration of the loading fixture shows parts in exploded view: The upper cup (1) holds the potted femur and the lower cup (2) holds the potted tibia of knee sample. The upper cup (1) is free to float between the vertical guides (3), allowing compression and varus/valgus realignment. Vertical guides (3) are fixed to the lower cup and restrict flexion and rotation of the knee. A spherical bearing (4) transmits load from the hydraulic actuator and pivots to allow the varus/valgus realignment, but is not rigidly attached to the upper cup (1). Load is sensed through the load cell (5), which is fixed to the load frame and the lower cup (2).
F3-30
Fig 3:
A photograph of the lateral aspect of the knee illustrates how the sample is aligned in the loading fixture. A sliding fit between the potting cement and cups, and a sliding fit between the upper cup and vertical guides allows the sample to be removed from the loading fixture for surgery, and then quickly and reliably replaced in the same alignment.

We used pressure-sensitive film (Pressurex®, SPI, East Hanover, NJ) to measure contact area and pressures on the lateral tibial plateau. Testing was conducted using super-low, low, and medium-range film. All three ranges of film were required to capture the full range of pressures between 0.5 MPa and 14 MPa. Film samples were placed in polyethylene packets to protect against moisture and inserted below the lateral meniscus. The packets consisted of envelopes of 0.018-mm-thick LDPE film. Knees were loaded to 1023 N (approximately double body weight) over 15 seconds, and held at load for 5 seconds. We tested four conditions: intact meniscus, horizontal tear, one leaf removed, and both leaves removed (Fig 4). Each knee was tested using each scale of film for three repetitions (nine loadings) in each of the four conditions. Testing was performed at room temperature, and knees were rinsed with phosphate-buffered saline between test conditions to prevent drying.

F4-30
Fig 4A:
D. A digital scan shows pressure-sensitive film markings for an (A) intact meniscus, (B) horizontal tear in the posterior third, (C) superior leaf removed, and (D) both leaves removed. The film was super-low range. The anterior of knee is toward the bottom of the image. The decreases in contact area after single- and double-leaf resection are evident.

In the first cohort of knees, with tears in the posterior third of the meniscus, the intact meniscus was tested with all ligaments intact. To create horizontal tears and perform resections, collateral ligaments were cut to gain access to the meniscus, and then were sutured before loading. The collateral ligaments were not strained during compressive testing, indicating the integrity of the collateral ligaments should have no influence on the results of this study. To create the horizontal tear, we made an incision from the tapered inner edge of the meniscus to within 2 mm of the meniscal rim (Fig 5). Half the knees had the superior leaf resected (Fig 6), and half had the inferior leaf resected for single-leaf resection. We randomly chose the four knees for superior-leaf resection at the outset of the study. Two millimeters of meniscal rim was left intact in the single-leaf and double-leaf resections.

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Fig 5:
A photograph shows a horizontal tear in the posterior third of the meniscus, created with a scalpel. The tear extends to within 2 mm of the meniscal rim.
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Fig 6:
A photograph shows the superior leaf of horizontal tear in the posterior third is resected. The resection spares the inferior leaf and 2 mm of the meniscal rim.

In the second cohort of knees, with tears involving the full extent of the meniscus, all ligaments were left intact during testing. The horizontal tear was simulated by using a scalpel to cut from middle height of the outer meniscus rim, exiting the inferior surface of the meniscus within 2 mm of the inner edge (Fig 7). The cut spanned the length of the entire meniscus between the anterior and posterior attachments. As in the first cohort, half of the knees had the superior leaf removed and half had the inferior leaf removed.

F7-30
Fig 7:
An illustration indicates how the cut for creating a horizontal tear through the cross-section of the meniscus is made from the rim of the meniscus to within 2 mm of the inner edge, exiting the inferior surface. This full-extent tear will extend the length of meniscus between the attachments.

We measured three parameters of interest: contact area, mean contact pressure, and peak contact pressure. The films were scanned in 256-level grayscale at 100 dpi (Microtek ScanMaker III, Carson, CA). To obtain local pressure measurements, we constructed a calibration curve relating grayscale to pressure. Data of average grayscale values obtained with a flat indenter (22.9-mm diameter) were used to construct a third-order polynomial describing the relationship between grayscale in 256 levels and pressure. Scans of test measurements were analyzed in MATLAB (The MathWorks, Inc, Natick, MA) using the image processing and statistical toolboxes. Grayscale values were converted to pressures, and a contact area was determined by specifying a threshold grayscale value corresponding to the lower limit of film sensitivity. To exclude noise from the calculation of contact area, a five-pixel by five-pixel averaging filter was applied to the image before applying the threshold. The contact area was measured using super-low-range film. The mean pressure was calculated as the area-weighted average of pressures from mutually exclusive threshold areas from each of the three ranges of film.21 Peak pressure was calculated as the maximum pressure in the threshold area. Low- and medium-range film gave comparable measurements of peak pressure between 10 MPa and 11 MPa, so if peak pressures were less than 11 MPa, or not indicated on medium-range film, the peak pressure was obtained from low-range film. Percentages were based on the mean for the intact meniscus: contact area of 105 (SE, 5) mm2, mean contact pressure of 4.3 (SE, 0.3) MPa, and peak contact pressure of 9.4 (SE, 0.4) MPa (SE indicates standard error).

For each parameter (contact area, mean pressure, peak pressure) in each cohort (full extent tear, posterior third tear), a mixed model analysis of variance (ANOVA) was performed with meniscus condition as the fixed effect and knee sample as the random effect. Multiple comparisons were made between meniscus conditions, and the Tukey-Kramer correction was used in constructing 95% confidence intervals (CI) to test for differences. To answer our first two research questions, we examined comparisons between single- and double-leaf resections for the two cohorts. To examine the extent to which sparing a leaf maintains meniscal function, we expressed results as a percentage, interpolated between an upper bound representing full function, and a lower bound representing no function. The mean value of contact area, mean pressure, and peak pressure for the intact knee was taken as the upper bound and keyed to 100% function. CI indicated 95% confidence interval, and percentages indicated interpolation between upper and lower bounds corresponding to values to the intact and meniscectomy limits. The mean value for double-leaf resection of a full-extent tear (in essence a total meniscectomy) was taken as the lower bound and keyed to 0% function.

RESULTS

For horizontal tears involving the posterior third of the meniscus, sparing one leaf provided a biomechanical benefit over resection of both leaves, as indicated by greater contact area and lower mean pressure (Tables 2 and 3). Sparing one leaf provided a 15% greater (p = 0.018) contact area, and a 27% lower (p = 0.009) mean contact pressure. However, peak contact pressures were similar. Although it provided a benefit over double-leaf resection, single-leaf resection did compromise biomechanical function of the meniscus when compared to the intact case: single-leaf resection decreased (p < 0.001) contact area by 40% and increased (p = 0.021) mean pressure by 24%. However, there was no change in peak pressure.

T2-30
TABLE 2:
Means of Contact Area and Pressure, Posterior-third Tears
T3-30
TABLE 3:
Comparison of Contact Area and Pressure, Posterior-third Tears

For horizontal tears spanning the extent of the meniscus, sparing one leaf provided no benefit over resection of both leaves, as indicated by similar contact area and pressure measurements from resection of both leaves (Tables 4 and 5). For full-extent tears, single-leaf resection compromised biomechanical function compared with the intact meniscus: single-leaf resection decreased (p < 0.001) contact area by 59%, increased (p < 0.001) mean pressure by 55%, and increased (p = 0.001) peak pressure by 18%.

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TABLE 4:
Means of Contact Area and Pressure, Full-extent Tears
T5-30
TABLE 5:
Comparisons of Contact Area and Pressure, Full-extent Tears

Sparing one leaf of a posterior third tear maintained a portion of the load-distributing function of the intact meniscus: normalized values of meniscal function were 37% (CI, 15%) for contact area, 55% (CI, 20%) for mean pressure, and 46% (CI, 39%) for peak pressure (Fig 8). Sparing one leaf for tears that span the full extent of the meniscus did not preserve meniscal function substantially above that of a total meniscectomy, as indicated by the normalized area and pressure measurements. Upper bounds at 95% confidence for the benefit of single-leaf resection in a full-extent tear were 29% for contact area, 15% for mean pressure, and 40% for peak pressure.

F8-30
Fig 8:
A bar chart indicates only single-leaf resection of a horizontal tear in the posterior third of the meniscus retains a significant amount of function. For measurements of contact area, mean pressure, and peak pressure, 100% is keyed to mean values for the intact meniscus, and 0% is keyed to mean values for total meniscectomy (double-leaf removal of full extent tear). Error bars indicate 95% confidence intervals. Significant difference (p < 0.05) from total meniscectomy is indicated by (**), and significant difference (p < 0.05) from double-leaf removal in a posterior third tear is indicated by (*).

We found no differences between resection of the inferior and superior leaves although these tests included only four unpaired subjects per sample. A more powerful study might detect such differences, although they would be small relative to the difference observed between single- and double-leaf resections for a posterior third tear.

DISCUSSION

We asked whether sparing one leaf of a horizontal meniscus tear offers a biomechanical benefit over resection of both leaves, as indicated by increased contact area and decreased contact pressure. We also sought to determine if the extent of the tear influences the benefit of sparing a single meniscal leaf. Our study allowed us to quantify the degree of preservation of meniscus function after tearing, resection of one leaf, and resection of both leaves.

The data indicate that, for posterior third tears, sparing one leaf provides a biomechanical benefit over resection of both leaves, and preserves nearly half of the load-distributing function of the meniscus. Alternately, for extensive tears, sparing one leaf provides little to no biomechanical benefit over resection of both leaves, and maintains little to no load-distributing function of the meniscus.

These tests were conducted on cadaveric sheep knees rather than human knees. However, the sheep is a well-established model for meniscectomy-induced OA, and the pattern and progression of OA in sheep after meniscectomy has been found to mirror the pattern and progression of OA in humans after meniscectomy.27,28 Although the precise changes in contact area and pressure may differ between humans and sheep, we believe the general conclusions drawn from this study are applicable to the human situation. The tests were conducted only at one angle of extension, the neutral position for the sheep knee; however, it is this angle we expect most relevant because it is the position at which the greatest loads are placed on the knee.37 Additionally, in a similar study of human knees by Huang et al,21 conclusions regarding differences in contact area, mean pressure, and peak pressure between intact, meniscectomized, and knees with allografts did not change for measurements over a range from neutral position to 45° flexion. Because of the serial nature of the surgeries, the order of testing each condition could not be randomized. However, the effect of test order did not seem important because no systematic evolution of results was observed for the three repetitions that were performed at each condition. Intermediate extents of meniscal resection, the effect of the size of rim left, and the effects of shear from sliding or twisting were not studied, as these topics were outside the scope of the study. Finally, the knees used in this study were from young animals with good meniscal tissue, and the tears were created with a scalpel, rather than through a degenerative process as is usually the case with horizontal meniscus tears. The performance of the menisci in this study thus represents the best-case scenario after a horizontal tear and resection. If the meniscus tissue is degenerative, the observed biomechanical advantages to sparing one leaf of a posterior-third horizontal meniscus tear could be negated.

The Pressurex® pressure sensitive film (also known as Fuji film) we used has been popular for assessing joint contact.5,18,21,22,40 The film is sensitive to moisture and temperature, so we sealed the film in polyethylene packets and tested in a temperature-controlled lab. The film is susceptible to wrinkle artifacts that inaccurately raise pressure when used on doubly curved surfaces; testing on the flatter lateral tibial plateau mitigated this issue and wrinkle artifacts were not observed in the data. Wu et al40 suggested inserting the 0.3-mm film in a joint could increase estimates of contact pressure by 10% to 26%, although we do not believe this affects our conclusions as relative comparisons between conditions still would be valid. We also verified, through trial insertions and removals, that the process of inserting the film beneath the meniscus did not cause artifacts. Pressurex film has a limited range, hence super-low, low-, and medium-range film were tested at each condition and the results combined to capture the relevant range of pressures in this study from 0.5 MPa to 14 MPa. The method of combining results from multiple ranges of film was inspired by that used for two ranges of film by Huang et al.21 Pressurex film results has been compared with a system that offers real-time measurement and potential for improved accuracy in an array of sensels known as KScan (TekScan, Inc, South Boston, MA).4,6,14,15,17 Although custom production of a KScan system appropriately shaped for sheep knees is possible, using three ranges of Pressurex film was deemed to have sufficient accuracy and flexibility for this study.

In examining pressure results, one might ask how large increases in mean pressure can be accompanied by smaller increases in peak pressure. We believe this variation occurred because of a change in shape of the pressure distribution (Fig 9). A typical pressure distribution for an intact meniscus is approximately triangular, while the pressure distribution after meniscectomy, with a smaller contact area, is approximately elliptical. The changing shape of the pressure distribution results in changes in mean pressure and peak pressure that are not proportional.

F9-30
Fig 9:
A plot of contact pressure from one sample illustrates how the change in shape of the contact pressure distribution after meniscectomy results in large changes in mean pressure, but small changes in peak pressure. The pressure distribution for the intact meniscus is approximately triangular, whereas that after meniscectomy is more elliptical.

A direct comparison of our results with results of previous investigations of tibiofemoral contact pressure is complicated by the use of different subject types, resection types, measurement systems, and loading magnitudes. However, comparing magnitudes of contact pressure and relative changes in area and pressure between the intact and meniscectomized cases, while noting the experimental differences, can be informative. Ahmed and Burke1 used a plastic indentation transducer to measure pressure in human knees and found peak pressures of 2.75 MPa with a load of 890 N, although this load is much less than the peak load during walking used in our study. Baratz et al,5 in a study of human knees loaded to 1800 N, using only one range of film, reported peak pressures of approximately 2 MPa in intact knees, 3 MPa to 4 MPa with segmental meniscectomy, and 6 MPa in completely meniscectomized knees. Pressure measurements were made in six locations with a Fuji densitometer over discrete areas, which likely underestimate peak pressures compared with pressure reported in later studies where optical scanners were used to measure film intensity. Reductions in contact area of 60% to 75% were shown with segmental and total meniscectomies, which is close to the reductions in contact area of 59% and 63% seen with single- and double-leaf resection of full-extent tears in the present study. Ihn et al22 studied human knees loaded to 600 N, 1200 N, 1800 N, and 2400 N and measured contact area with low-range, pressure-sensitive film. They found 33% to 55% reductions in contact area in knees loaded to 1200 N. Huang et al21 performed a study of the lateral human meniscus with measurement techniques similar to those we used, although loaded to 400 N and 1200 N, and used only super-low and low-range film. At 1200 N, peak pressures were near 5.5 MPa for intact knees and 8 MPa for meniscectomized knees, mean pressures were 2.5 MPa for intact versus 5 MPa for meniscectomized knees, and contact area reduced by only 30% with meniscectomy. Because of loading differences, direct comparison with our results is difficult, and one of the findings by Huang et al was non-linearity of the scaling of pressures and contact areas between load levels. Lee et al25 studied radial partial and total meniscectomy of human knees loaded to 1800 N with measurement performed by a TekScan system. Peak pressures of 4.5 MPa and 9.5 MPa, mean pressures of 2 MPa and 3.5 MPa, were reported for intact and meniscectomized knees, with a 52% reduction in contact area. Their study showed progressive degradation of load-distributing function with increasing amounts of radial resection, and that segmental meniscectomy was comparable to total meniscectomy. Pressure values are somewhat lower than in our study, and changes in contact area are comparable. The somewhat higher pressures in the current study could be attributed to differences in specimen type, the meniscus studied, and differences between the applied load and actual gait load for each specimen. In each of these studies, the relative decreases in contact area are similar, which indicates the meniscus carried similar amounts of load.

The findings of the current study, regarding horizontal partial meniscectomy, mesh with the findings of other studies of radial and segmental meniscectomies. Considered together, the findings suggest any resection of meniscal tissue involves substantial compromises to meniscal function. The studies also confirm resections that eliminate the ability of the meniscus to carry hoop stresses, whether by cutting vertically through the meniscus as with segmental meniscectomy, or by thinning it with resection of one leaf of an extensive horizontal tear, will result in a biomechanical condition similar to that of total meniscectomy. The changes in data from biomechanical studies, considered in light of those from clinical studies of partial meniscectomy,10,11 are consistent with changes in contact area and contact pressure relating to changes in severity of OA symptoms. However, the quantitative mapping between a particular biomechanical parameter and increased risk of OA symptoms has not been established.

For common horizontal tears in the posterior third of the meniscus, sparing one leaf offers a biomechanical benefit over resection of both leaves, maintaining approximately half the intact meniscus function. Considering clinical findings that correlate increased resection with negative OA outcomes, particularly for degenerative tears,11 and infer a link between joint contact parameters and OA progression, we conclude sparing one leaf in posterior-third tears may lead to better patient outcomes than resection of both leaves. For extensive horizontal meniscus tears, sparing one leaf provides minimal or no biomechanical benefit compared with resecting both leaves.

Acknowledgments

We thank Derek Lindsey for assistance with the test apparatus, and JJ&F Market for providing the tissue specimens.

References

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