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First published on November 10, 2005, doi:10.1177/0363546505281236
This version was published on January 1, 2006
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The American Journal of Sports Medicine 34:116-119 (2006)
© 2006 American Orthopaedic Society for Sports Medicine

The Effect of Angled Osteochondral Grafting on Contact Pressure

A Biomechanical Study

Jason Lee Koh, MD*,{dagger}, Adam Kowalski{ddagger} and Eugene Lautenschlager, PhD{dagger}

From the {dagger} Northwestern University Feinberg School of Medicine, Chicago, Illinois, and {ddagger} Indiana University School of Medicine, Indianapolis, Indiana

* Address correspondence to Jason Lee Koh, MD, Department of Orthopaedic Surgery, Northwestern University Medical School, 645 North. Michigan Avenue, Suite 910, Chicago, IL 60611 (e-mail: kohj1{at}hotmail.com).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: Flush osteochondral plugs can reduce contact pressure compared with an empty defect in the articular cartilage. However, incongruities such as graft angulation have an unknown effect.

Hypothesis: Incongruity of the articular cartilage after osteochondral transplantation affects articular surface contact pressure.

Study Design: Controlled laboratory study.

Methods: An 80-N load was applied with a material testing system for 120 seconds to the femoral condyles of 50 fresh swine knees. Contact pressures were measured using Prescale super low film. Five conditions were tested: (1) intact articular surface; (2) surface with 4.5-mm-diameter circular defect; (3) defect grafted with a flush 4.5-mm-diameter plug from the contralateral condyle; (4) defect grafted with a 30° angled 4.5-mm-diameter plug, with lower edge flush (tip elevated with respect to the adjacent surface); and (5) defect grafted with a 30° plug, with tip flush to the adjacent surface (lower edge sunk). Angled grafts were obtained using a rotational bearing vise aligned with a 30° fixed-angle track. The film was digitally scanned and analyzed, and standard statistical tests were performed.

Results: Mean peak pressures of intact cartilage (8.57 kg/cm2), flush graft (9.81 kg/cm2), and sunk and angled graft (9.15 kg/cm2) were not significantly different (P < .5). The mean pressures for defects (12.01 kg/cm2) and the elevated angled graft (14.50 kg/cm2) were significantly (P < .05) higher than that of intact cartilage.

Clinical Relevance: Slightly sunk grafts were still able to reduce elevated contact pressures to normal levels. However, elevated angled grafts increased contact pressure. These results suggest that it is preferable to leave an edge slightly sunk rather than elevated.

Key Words: osteochondral transplant • contact pressure • biomechanics • cartilage transplant


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Articular cartilage lesions may lead to significant pain and disability.8,10 Osteochondral autografting has become increasingly popular as a means of treating articular cartilage defects.1,3,5 This technique involves the transplantation of osteochondral plugs from relatively non-weightbearing areas of the knee to an area of cartilage injury. Osteochondral autografting has the advantage of transplanting intact articular cartilage, which has been demonstrated to retain normal biomechanical properties at initial implantation.7 It has been previously shown that a single plug placed flush can reduce elevated contact pressures to normal levels and reproduce normal contact pressure patterns.7 However, small articular surface incongruities may occur after osteochondral plug grafting. We investigated the effect of angled grafts on femoral condyle articular cartilage defects on contact pressure in the knee.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Fifty fresh knees were harvested from adult swine (age, 3 months; mean weight, 70 kg). The thickness and biomechanical properties of the articular cartilage of swine closely correspond to the cartilage of human knees in pilot and previous studies.7 Soft tissue from the knees was removed, and the knees were frozen until testing, when they were thawed for 4 to 6 hours at 25°C and mounted on an Instron material testing system (Instron, Canton, Mass).

Articular Surface Conditions
Five conditions were tested: (1) intact articular surface; (2) surface with 4.5-mm-diameter circular defect; (3) defect grafted with a flush 4.5-mm-diameter plug from the contralateral condyle; (4) defect grafted with a 30° angled 4.5-mm-diameter plug, with the lower edge flush (tip elevated with respect to the adjacent surface); and (5) defect grafted with a 30° plug, with the tip flush to the adjacent surface (lower edge sunk). Ten knees were tested in each group, with 3 repetitions of each test.

Intact Specimen Measurements
A 4.5-mm-diameter circular defect was delineated with indelible marker on the articular surface of the distal medial femoral condyle, and the donor site on the lateral femoral condyle was similarly delineated. The size was chosen as a representative size of a defect that might be clinically treated and in proportion to the size of the swine femoral condyle. The specimens were kept moist with normal saline throughout testing.

A constant load of 80 N was applied to the region via a loading piston for 120 seconds with interposed Fuji Prescale super low pressure-sensitive film (Fuji Photo Film Co Ltd, Tokyo, Japan) to measure baseline pressures and contact area profiles of the femoral condyle.2,7,11 This load was selected on the basis of the calculated relative normal weightbearing pressures believed to be applied to the joint by the swine limb during walking. The pressure-sensitive film was protected from moisture with a clear adhesive plastic film, as has been previously described.14

Defect Creation and Pressure Measurements
A circular full-thickness articular cartilage defect, 4.5 mm in diameter, was created at the previously delineated site (Figure 1Go). The underlying subchondral bone was left intact, and contact pressure measurements were duplicated with identical loading conditions. The donor and graft site were chosen to match articular cartilage thickness and orientation as closely as possible to eliminate variability from different donor sites.



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  		Figure 1. Schematic of graft harvest and transfer.

 
Grafting of Defect: Flush
The defect site was drilled perpendicularly to a depth of 12.5 mm. With the use of a tubular chisel, an osteochondral plug (4.5 mm x 12.5 mm) was perpendicularly harvested from the contralateral condyle and press-fit into the drilled-out defect so that it was flush with respect to the surrounding articular cartilage. The perpendicular harvest of the plug was conducted by visualizing the sides of the harvesting tool, which has laser markings running circumferentially around the device. These markings were parallel to the surface of the articular cartilage while the graft was harvested. In addition, the surface of the cartilage can be lightly touched with the harvesting tool, and a perfect circular mark should indicate a perpendicular graft (an obliquely harvested graft will produce an elliptical mark). The donor site was prepared in a similar fashion.

Grafting of Defect: Angled
A custom-designed apparatus (Figure 2Go) was used to obtain a graft with a 30° angle to the articular surface. This angle was chosen after the evaluation of a postoperative MRI of grafted osteochondral defects performed in a clinical setting. Using a 6 degrees of freedom jig, the articular surface of the contralateral condyle was placed so that the flat plate of the guide was against the condyle. This plate had been machined to allow the 4.5-mm tubular chisel to pass through at a 30° angle to the perpendicular. The tubular chisel was impacted until the lower edge of articular cartilage reached the 10-mm mark. This device allowed the consistent harvesting of grafts that had a 30° angle to the articular surface (Figure 3Go). This degree of angulation was based on clinical observations intraoperatively and MRI data from clinical follow-up as a reasonable amount of incongruity. The height difference at graft edges was sin(30) x 4.5 mm (=2.25 mm). The prominent grafts did not significantly subside during testing.



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  		Figure 2. Device to harvest angled graft.

 


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  		Figure 3. Side view of the elevated angled graft.

 
Analysis of Data
Each piece of film was labeled, digitally scanned, and analyzed using Scion Image computer software (Scion Corp, Frederick, Md). Load patterns were assessed, and contact pressures were derived from density data standardized to known values. Each of the 5 conditions was analyzed using repeated measures, analysis of variance, and multiple comparison post hoc testing (Student-Newman-Keuls).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Mean pressure and mean force over a standard area did not significantly change, indicating that similar total loads were seen by the pressure-sensitive film under each set of conditions. This result was expected, given the constant load applied by the Instron machine.

Peak contact pressures were significantly (P < .001) elevated after defect creation (intact, 8.57 ± 2.35 kg/cm2; defect, 11.97 ± 1.89 kg/cm2) and were reduced to near-normal values (9.81 ± 1.01 kg/cm2) (P < .001) when the plug was placed flush to the surrounding articular surface.

When implanted with the lower edge flush (higher edge elevated with respect to the adjacent articular cartilage), 30° angled plugs demonstrated significant (P < .001) increases in contact pressure compared with intact normal cartilage (Figure 4Go). These increases were quite high (14.50 ± 2.21 kg/cm2) (P < .001).



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  		Figure 4. Pressure-sensitive film after testing of sunk angled plug. Note gradual decrease in the optical density (pressure) as the surface of plug goes below the adjacent cartilage and has less load.

 
Contact pressures with the angled plug implanted with the upper edge flush to the adjacent cartilage (9.15 ± 3.34 kg/cm2) (P = .497) were not significantly different from those with intact cartilage or a flush plug. They were significantly less than those with the empty defect (P < .001).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The goal of articular cartilage repair procedures is to diminish morbidity and restore the normal biologic and biomechanical properties of intact articular cartilage, with the ultimate goal of restoring normal joint function. One procedure that is commonly used is osteochondral grafting of the defect.

In this model, we noted significant variations in contact pressure patterns and a statistically significant increase in peak pressure when a full-thickness articular cartilage defect was created on the femoral condyle. Brown et al2 and Nelson et al11 similarly demonstrated that circular defects in canine knees can cause an increase in peak pressures of 10% to 30%, with large changes in what they termed the gradient of pressure.

The increased peak pressure adjacent to an articular cartilage defect may cause overload of the tissue, resulting in pain from increased subchondral pressure or cartilage damage. In addition, the irregular pressure pattern may contribute to significant shear forces of overloaded tissue. This result has been demonstrated previously in articular cartilage incongruity models examining the effects of intra-articular step-offs in fractures.9

In this in vitro study, the treatment of an articular cartilage defect with a flush osteochondral plug graft re-created normal contact pressure patterns and significantly decreased the peak pressure to normal levels immediately after treatment. This restoration of normal biomechanical forces with osteochondral tissue may be the reason for the clinical effectiveness of osteochondral grafting in the knee.

However, there were striking (40%) increases in peak pressure when the plug was angled with an edge placed higher than the adjacent articular cartilage. This finding suggests that leaving a plug elevated may be biomechanically disadvantageous. This finding may also explain why in vivo plugs placed elevated demonstrate poor integration with surrounding articular cartilage when compared with those plugs placed flush to the adjacent tissue.4,6,12,13 In vivo, some additional compression has been seen to occur. Pearce et al13 noted that 2 of 6 elevated plugs remained elevated, and 1 had subsided. In addition, 5 of 6 elevated plugs had subchondral cavities that were thought to compromise subchondral support for the plugs.

Significant subsidence did not occur with the elevated angled grafts in this study. This result is most likely because the forces needed for implantation of the graft are significantly higher (200–800 N) than those used in this study (Butcher et al, unpublished data, 2004). We would anticipate that even higher loads would be needed for significant subsidence of an angled plug because the force would need to be delivered on only the elevated area, a potential source for further degeneration.

An angled graft with the highest edge placed flush to neighboring cartilage demonstrated near-normal contact pressures. This finding suggests that clinically, if minor angulation of the graft is noted, it is preferable to place the plug with the highest edge at the level of the adjacent articular cartilage.

There are a number of limitations to this study. The defect was filled with the ideal graft in this model. The selection of a graft from the central portion of the opposite weightbearing condyle was chosen to minimize variation in articular cartilage thickness and properties but does not duplicate the typical donor site, which has more variable properties. Only a single plug was used, with only a fixed degree of incongruity. This degree of angulation was based on clinical observations intraoperatively and MRI data from clinical follow-up as a reasonable amount of incongruity. This study does not duplicate the dynamic in vivo environment, in which the plugs undergo shear forces and potential remodeling. These shear forces on the elevated edge of grafts may result in softening of articular cartilage and possible enzymatic degradation and may possibly contribute to the poor integration with surrounding tissue.13 We have clinically seen via follow-up MRI and arthroscopy that slightly countersunk edges of plugs often undergo a proliferation of fibrocartilage repair tissue to match the surrounding articular surface, which may further redistribute load.

In conclusion, the elevated edge of an angled osteochondral plug can lead to significantly increased contact pressures that can be higher than those seen with the initial defect. This finding reinforces the importance of articular surface congruity in the initial biomechanical state after osteochondral implantation, and we recommend that meticulous attention be paid to the perpendicular preparation of the graft site and the harvesting of grafts. If an angled graft is inadvertently harvested, placing the plug with the highest edge flush with the surrounding articular cartilage may reduce initial contact pressures.


   FOOTNOTES
 
No potential conflict of interest declared.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Bobic V. Arthroscopic osteochondral autograft transplantation in anterior cruciate ligament reconstruction: a preliminary clinical study. Knee Surg Sports Traumat Arthrosc. 1996;3:262–264.[CrossRef][Medline][Order article via Infotrieve]
  2. Brown TD, Pope DF, Hale JE, Buckwalter JA, Brand RA. Effects of osteochondral defect size on cartilage contact stress. J Orthop Res. 1991;9:559–567.[CrossRef][ISI][Medline][Order article via Infotrieve]
  3. Hangody L, Kish G, Karpati Z, Szerb I, Udvarhelyi I. Arthroscopic autogenous osteochondral mosaicplasty for the treatment of femoral condylar articular defects: a preliminary report. Knee Surg Sports Traumatol Arthrosc. 1997;5:262–267.[CrossRef][Medline][Order article via Infotrieve]
  4. Hangody L, Kish G, Karpati Z, et al. Autogenous osteochondral graft technique for replacing knee cartilage defects in dogs. Orthopedics Int Ed. 1997;20:525–538.[Medline][Order article via Infotrieve]
  5. Hangody L, Kish G, Karpati Z, Szerb I, Udvarhelyi I. Mosaicplasty for the treatment of articular cartilage defects: application in clinical practice. Orthopedics. 1998;21:751–756.[ISI][Medline][Order article via Infotrieve]
  6. Hurtig M, Evans P, Pearce S, Clarnette R, Miniaci A. The effect of graft size and number on the outcome of mosaic arthroplasty resurfacing: an experimental model in sheep. In: Transactions, 18th Annual Meeting Arthroscopy Association of North America; Vancouver, British Columbia, Canada; 1999; 16.
  7. Koh JL, Kambic H, Valdevit A, Petty D, Bergfeld JA, Dimitris K. Osteochondral autografting of an idealized articular cartilage defect: in vitro evaluation. In: Transactions, Orthopaedic Research Society; San Francisco, California; 2001;26:827.
  8. Levy AS, Lohnes J, Sculley S, LeCroy M, Garrett W. Chondral delamination of the knee in soccer players. Am J Sports Med. 1996;24:634–639.[Abstract/Free Full Text]
  9. Llinas A, McKellop HA, Marshall GJ, Sharpe F, Kirchen M, Sarmiento A. Healing and remodeling of articular incongruities in a rabbit fracture model. J Bone Joint Surg Am. 1993;75:1508–1523.[Abstract/Free Full Text]
  10. Messner K, Maletius W. The long-term prognosis for severe damage to weight-bearing cartilage in the knee: a 14-year clinical and radiographic follow-up in 28 young athletes. Acta Orthop Scand (Norway). 1996;67:165–168.
  11. Nelson BH, Anderson DD, Brand RA, Brown TD. Effect of osteochondral defects on articular cartilage: contact pressures studied in dog knees. Acta Orthop Scand (Denmark). 1988;59:574–579.
  12. Paletta GA, Ibarra C, Wannafin J, Potter HG, Torzilli P. Histologic, biomechanical, and MR image evaluation of autogenous osteochondral plug transplantation using a dog model. In: Transactions, Cartilage Study Group Meeting; New Orleans, Louisiana; March 21, 1998; 22.
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