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. Author manuscript; available in PMC: 2009 Jan 1.
Published in final edited form as: Osteoarthritis Cartilage. 2008 Jan 11;16(7):841–845. doi: 10.1016/j.joca.2007.11.015

Wear-Lines and Split-Lines of Human Patellar Cartilage: Relation to Tensile Biomechanical Properties

Won C Bae *, Van W Wong *, Jennifer Hwang *, Jennifer M Antonacci *, Gayle E Nugent-Derfus *, Megan E Blewis *, Michele M Temple-Wong *, Robert L Sah *,#
PMCID: PMC2613195  NIHMSID: NIHMS59220  PMID: 18248747

Abstract

Background

Articular cartilage undergoes age-associated degeneration, resulting in both structural and functional biomechanical changes. At early stages of degeneration, wear-lines develop in the general direction of joint movement. With aging, cartilage exhibits a decrease in tensile modulus. The tensile modulus of cartilage has also been related to the orientation of the collagen network, as revealed by split-lines.

Objective

To determine the relative contribution of wear-line and split-line orientation on the tensile biomechanical properties of human patellar cartilage from different depths.

Methods

In human patellar cartilage, wear- and split-lines are aligned parallel to each other at the proximal facet, and perpendicular to each other at the medial facet. Using superficial, middle, and deep cartilage sections from these two sites, tensile samples were prepared in two orthogonal orientations. Thus, for each depth, there were four groups of samples, with their long axes were aligned either parallel or perpendicular to wear-line direction and also aligned parallel or perpendicular to split-line direction. Uniaxial tensile tests were performed to assess equilibrium and ramp moduli.

Results

Tensile equilibrium modulus varied with wear-line orientation (p<0.05) and depth (p<0.001), in an interactive manner (p<0.05), and tended to vary with split-line orientation (p=0.16). In the superficial layer, equilibrium and ramp modulus was higher when the samples were loaded parallel to wear-lines (p<0.05).

Conclusion

These results indicate that mild wear (i.e., wear-line formation) at the articular surface has deleterious functional effects on articular cartilage and represents an early aging associated degenerative change. The identification and recognition of functional biomechanical consequences of wear-lines is useful for planning and interpreting tensile biomechanical tests in human articular cartilage.

Keywords: Tensile, Modulus, Degeneration, Pathogenesis, Wear, Split-line

Introduction

Articular cartilage is a wear-resistant connective tissue that covers ends of long bones, allowing normal joint motion. The dependence of biomechanical properties of cartilage on structure and composition is of interest for understanding its function in health as well as its deterioration in aging and disease. In particular, the collagen network in cartilage bears tensile loads that develop during joint contact1, but is susceptible to disruption with aging, as evidenced by surface fibrillation and wear-lines 2-4. Such degeneration at the articular surface has been associated with a reduction in tensile biomechanical integrity5-8.

The collagen network at the articular surface has a preferred orientation, and can be revealed by split-lines at the surface (Fig. 1A) that vary characteristically between and within joints9-13. When the articular cartilage surface is pricked with an ink-dipped pin to the depth of subchondral bone, the tissue splits parallel to the main direction of the collagen network14. In the human patella, split-lines (Fig. 1A) are aligned in a proximal-to-distal direction over most of the surface (including the proximal facet, Fig. 1B), but in a lateral-to-medial direction on the lateral facet (Fig. 1C)9. Additionally, the collagen network structure varies with depth from the surface, being predominantly parallel to the articular surface in the superficial zone, turning obliquely in the middle zone, and being vertical to the bone in the deep zone, anchoring into the calcified cartilage14,15.

Figure 1.

Figure 1

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(A) Split-lines (arrows) in human patellar cartilage are visible after pricking the articular surface with a conical awl, and oriented in (B) lateral-to-medial and (C) proximal-to-distal directions in the proximal and the lateral facets, respectively.

Tensile biomechanical properties of cartilage have been related to variations in the direction of loading, relative to the split-line orientation. Due to the thinness of articular cartilage (~mm in human joints), specimens for tensile testing are prepared by slicing the tissue parallel to the articular surface (which controls the depth from which the sample is obtained) and stamping out a tapered sample (e.g., Fig. 3) whose long axis defines the axis of loading. Tensile samples aligned either parallel or perpendicular to the split-line direction exhibit relatively high or low tensile integrity, respectively, when taken from adult animals13,16,17 and humans18. In addition, tensile integrity decreases with depth from the articular surface, being highest in the superficial layer and lower for the deeper layers16,18.

Figure 3.

Figure 3

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(A) Tensile samples can be prepared to yield four unique orientations with respect to the split-line and the wear-line orientations. (B) Predicted outcomes assuming dominance of either wear-line or split-line effects to lower tensile integrity.

Articular cartilage degeneration involves surface wear and disruption, whose extent and directionality varies with aging. Studies on morphological patterns of cartilage wear and fibrillation using India ink staining3,4,19 suggest that ~parallel lines of varying width begin to form on joint surfaces, which then progress to “overt fibrillations” or areas of increasingly confluent blackening without directionality, with advancing age. In the human patella, by age forty, ~30% of the articular surface is disrupted by parallel wear-lines4, oriented in the proximal to-distal direction. With further aging, wear-lines are less evident and replaced by overt fibrillation. The wear-lines on the articular surface may represent an early stage of cartilage degeneration and also have consequences for the way in which the tissue bears load.

Wear-lines may have a biomechanical effect on articular cartilage, alone or interactively with split-lines. Although split-line direction has been correlated with tensile properties in past studies13,16-18, the effect of wear-line orientation, which does not necessarily coincide with split line orientation19, is unknown. In the human patella, the proximal, medial, and distal facets exhibits wear-lines and split-lines that align parallel, and the lateral facet exhibits wear-lines and split-lines that align perpendicular to each other (Fig. 3A). From these regions, four types of tensile samples can be obtained (Fig. 3A). We hypothesize that wear-line and split-line direction will have independent and interactive effects on tensile properties of human articular cartilage, resulting in distinct levels of tensile integrity (Fig. 3B). Therefore, the objective of this study was to determine the relative contribution of wear-lines and split-lines on tensile biomechanical properties of human patellar cartilage from different depths.

MATERIALS AND METHODS

Samples

Patellae were obtained from cadaveric human donors (n=10, 38.6±4.2 yrs), from tissue banks. These were chosen based on a macroscopic surface appearance at the time of harvest that was either smooth and normal or slightly roughened, but without obvious erosion. The patellae were stored at -70°C until testing. An additional patella was also harvested pricked throughout the articular surface with an inked awl11,12 to create and visualize split-lines (Fig. 1).

India Ink Staining and Imaging

Patellae were thawed in a solution of phosphate buffered saline containing proteinase inhibitors20 (PBS+PI). The surface of each patella was painted with a solution (1:5) of India ink and PBS+PI, wiped to remove excess ink solution, and imaged (Fig. 2) using a digital imaging system21.

Figure 2.

Figure 2

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(A) Wear-lines (arrows) in human patellar cartilage are visible after India ink staining of the articular surface, and oriented in the proximal-to-distal direction throughout the surface of the patella, including (B) lateral and (C) proximal facets.

Topographical variations in split-line and wear-line orientations were verified on images. Split-line angle relative to the lateral-medial line, measured using ImageJ (NIH, Bethesda, MD) on photographs of the split-line sample (Fig. 1A) and that from a past study9, were 10.2±3.2° (inter-sample mean±standard deviation) and 86.9±2.6° for the lateral (Fig. 1C) and the proximal (Fig. 1B) regions of the patella, respectively. Remaining intact and stained patellae had wear-line angles of 95.4±11.7° and 88.1±8.1° in the lateral and the proximal regions, respectively. This was consistent with generally proximal-distal alignment of wear-lines, and regional variations in split-lines of the human patella.

Tensile Specimens and Testing

Tensile samples were prepared by extracting two osteochondral cores (11 mm diameter) from proximal and lateral aspects of patella (Fig. 3A) using a surgical coring device (Osteochondral Autograft Transfer System™, Arthrex Inc., Naples, FL). Cores were notched to maintain orientation. The thickness of articular cartilage of each core was measured using digital imaging, and found to be similar for all cores (~3.4±0.2 mm). Each core was sectioned parallel to the articular surface in ~300 μm thick increments using a sledge microtome (HM440E, Microm, Kalamazoo, MI) to obtain superficial (S, 0% depth, including the articular surface), middle (M, ~30%), and deep (D, ~60%) layer slices of cartilage. Using a tapered stamp, the slices were punched in either proximal-to-distal or lateral-to-medial orientation to generate four types of tensile samples, each with unique orientation with respect to wear-lines and split-lines (Fig. 3B). Each tensile specimen (gage region of 0.8×4×0.3 mm3, W×L×H) was secured between spring-loaded clamps, while minimizing the slack22. Samples were elongated to 10% and then 20%, with stress relaxation to equilibrium at each strain level23. The equilibrium modulus was computed as the slope of linear fit of the stress-strain data at equilibrium. Then the specimen was pulled (5 mm/min) until failure, and the ramp modulus was determined from linear portion of the data, between 25% and 75% of the maximum strain, to minimize the artifact due to nonlinearly of tensile behavior of cartilage22.

Statistics

First, repeated measures ANOVA was used to determine the effect of core location (proximal or lateral facets) and layer (S, M, or D) on tensile properties; Since there was no significant difference between locations (each p>0.6 and interaction p>0.5), repeated measures ANOVA (rmANOVA)was used to determine the overall effects of split-line direction (parallel or perpendicular to the specimen length and axis of loading), wear-line direction (parallel or perpendicular), and layer (S, M, or D) on tensile properties. Further, because large differences between layers were expected18, two-way ANOVA (split-line and wear-line directions as factors) was performed at each layer. Data are presented as mean ± standard error of the mean (SEM).

RESULTS

Tensile equilibrium modulus (Fig. 4A) varied with wear-line orientation (rmANOVA p<0.05) and layer (rmANOVA p<0.001), in an interactive manner (rmANOVA p<0.05), and tended to vary with split-line (rmANOVA p=0.16). Tensile moduli decreased with depth, from highest values in the superficial layer to the lowest in the deep layer. In the superficial layer, equilibrium modulus was higher when the tensile sample orientation (i.e., axis of tensile loading) was parallel to the wear-line (p<0.05), while in the middle layer, the values tended to be higher when samples were parallel to the split-line direction (p=0.07). In the deep layer, properties did not vary significantly with wear-line or split-line direction of the sample.

Figure 4.

Figure 4

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(A) Equilibrium and (B) ramp moduli of four types of tensile samples, with unique directionality with respect to the split-line and the wear-line orientation. Mean ± SEM.

For the superficial samples, the order of groups with increasing equilibrium modulus (Fig. 4A, S) was consistent with the predicted outcome assuming a dominant effect of wear-line (Fig. 3C, top). Specifically, cross-directional (wear-lines are perpendicular to split-lines) samples loaded perpendicular to wear-lines had lower moduli than those loaded parallel to wear-lines. In contrast, the order of the middle layer samples (Fig. 4A, M) was consistent with the case in which the effect of split-line was dominant (Fig. 3C, bottom). For the middle layer cross-directional samples, loading perpendicular to split-line direction resulted in lower moduli than loading parallel to split-lines. Similar trends were found for ramp modulus (Fig. 4B), although some of the factors did not reach statistical significance.

DISCUSSION

The present study determined the relative contribution of wear- and split-lines on apparent tensile biomechanical properties of human articular cartilage. The results suggest that loading perpendicular to wear-lines result in lowered tensile modulus of the superficial zone cartilage (S, Fig. 4A), where a minor effect of split-lines (i.e., samples taken perpendicular to split-lines have lower moduli) was also detected. This trend was reversed in the middle layer (M, Fig. 4), where a minor tensile weakening effect of wear-lines was found, compared to that of the split-lines. This suggests that wear lines affect the tensile properties of cartilage specimens taken from the articular surface.

The present study utilized natural topographical variations in wear- and split-lines of the human patella as a natural experimental model. In human patellae, wear-lines occurred in the proximal-to-distal orientation (Fig. 1), consistent with past studies3,4. In contrast, split-line direction varied topographically (Fig. 2), being proximal-to-distal except at the lateral facet, also consistent with the literature9. Such difference in directionality of wear-line and split-line allowed for selection of appropriate tensile samples. While split-line direction has not been assessed in all of the present samples, the inter-sample variation in split-line orientation is small9, especially in the loaded regions12. In addition, the human joints have other regions where unique combinations of wear- and split-line directions likely exist. For example, split-lines of the articular surfaces of central patellar groove and femoral condyle are oriented parallel and perpendicular, respectively, to the direction of knee movement12 along which wear-lines are likely to develop with time. Determination of wear- and split-line patterns in other joints and their influence on biomechanical properties could also be done following the approach of the present study.

The biomechanical effects of wear- and split-line orientation are likely to have distinct structural bases. The effect of split-line orientation has been ascribed to the intrinsic anisotropic structure of the collagen network with an overall direction13,14,24 and collagen network being the stiffest and the strongest in tension, as suggested in compression-tension nonlinearity of cartilage25. For example, tensile loading perpendicular to the split-line orientation results in 1) fewer numbers of fibrils being recruited compared to a loading in the parallel direction, and 2) tensil1 e modulus13. In contrast, wear-lines represent the structural failure and physical disruption of the cartilage surface26. This is evidenced by a weak effect of wear-lines on tensile properties at the middle layer (M, Fig. 4), which is likely to be intact for samples with mild degeneration of the surface.

Geometric consideration of wear-lines as physical grooves has an implication for the mechanism of tensile weakening during cartilage degeneration. For example, if a sample contains a hypothetical groove, the depth or the width of the groove would dominantly affect the tensile properties when loaded perpendicular or parallel to the axis of the groove, respectively. Such consideration would seem consistent with the present results (i.e., strong influence of wear-lines at the surface), since mildly degenerate samples may have wear-lines resembling narrow and deep grooves, much like vertical clefts often seen in histopathological analyses of cartilage tissue27,28. In addition, wear-lines may contribute to the apparent diminution of tensile properties of the cartilage surface during aging5,29 or degeneration6,7, independent of changes in matrix composition. For instance, while the tensile modulus of the superficial layer samples of human patellar cartilage decreases with increasing adult age, that of the middle layer samples is relatively unchanged with age29. Similarly, while a fibrillated region of cartilage has a markedly lower tensile modulus than normal regions at the superficial layer, the middle layer of the fibrillated region may have similar or higher modulus than the corresponding layer of the normal region6. Such findings maybe due to structural damage at the articular surface associated with wear-lines. Mechanisms of the development of wear-lines remain to be established, but may involve a mechanical wear process such as abrasion30, adhesion, and fatigue31, and in concert with biochemical alterations driven by non-enzymatic or enzymatic reactions32,33.

Combined effects of wear- and split-lines also have implications for the pathogenesis and progression of cartilage degeneration. Since the tensile integrity is the weakest when loaded perpendicular to both split-lines and wear-lines, it would seem likely that such locations (i.e., where split- and wear-line orientations coincide) may have a relatively high prevalence for failure. In contrast, regions with split-lines and wear-lines are perpendicular to each other may have lowered predilection for injury and degeneration under tension. In the patella, the lateral most facet has such the latter type of structural features, and it has a relatively lower incidence of degeneration than most other parts of the patella34. Intrinsic orientation of wear- and split-lines in joints may be considered among many other factors of cartilage degeneration, such as over-loading35 and joint destabilization36.

The present results may also have practical implications, such as when planning an osteochondral graft. Focal defects of articular cartilage can be treated37-39 by insertion of osteochondral fragments. For successful outcome, factors such as re-establishment of original joint contour39, which has consequences for contact stress40, as well as matching of graft split-lines to those surrounding cartilage has been suggested11,12,41, the latter due to biomechanical effects of the split-lines. Since wear-lines also influence biomechanical properties at the superficial zone, matching of the normal direction of wear-lines may also be beneficial.

ACKNOWLEDGMENTS

This study was supported by grants from NIH, NSF, and HHMI.

Footnotes

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