Hexagonal grafts in mosaicplasty: Biomechanical comparison of standard cylindrical and novel hexagonal grafts in calf cadaver model.

OBJECTIVE: Cylindrical grafts are currently used to cover defected area in mosaicplasty. However, there are some difficulties with cylindrical grafts, such as potential dead space between grafts and insufficient coverage. Hexagonal graft (honeycomb model) was created and evaluated in this biomechanical study. Hypothesis was that harvesting grafts with hexagonal shape, which has the best volume geometry characteristics in nature, would be biomechanically advantageous and provide superior pull-out strength.
METHODS: Total of 24 fresh calf femurs were divided into 3 equal groups. In the first group, 1 cylindrical and 1 hexagonal graft were compared. Second group consisted of 3 cylindrical and 3 hexagonal grafts. Third group was designed to evaluate effect of graft depth; hexagonal graft implanted at 5 mm depth was compared with 20-mm-deep hexagonal graft. All specimens were subjected to pull-out test. Friction field and graft surface area were also evaluated.
RESULTS: Pull-out strength comparison of 15-mm-deep triple cylindrical grafts and 15-mm-deep triple hexagonal grafts in second group revealed statistically significant difference in favor of hexagonal grafts (p < 0.05). Surface area of cylindrical graft with 9-mm diameter was calculated to be 50.27 mm2, while hexagonal graft surface area was 55.425 mm2. Volume ratio of cylindrical and hexagonal grafts was 753.98 mm3 and 831.375 mm3, respectively.
CONCLUSION: This biomechanical study demonstrated that graft geometry, especially in multiple graft applications, is a factor that influences stability. Hexagonal grafts appear to be more stable than cylindrical grafts in multiple applications, and they may be used to cover a larger defected area.

The treatment of injuries to articular cartilage, which has very low potential for spontaneous repair, is a challenging problem for orthopedic surgeons. Several techniques, including abrasion arthroplasty, drilling, and microfracture, have been described to repair articular defects in weight-bearing areas of the knee.2, 3, 4 However, these techniques have led to fibrocartilage formation rather than normal articular hyaline cartilage. As a result, new procedures for hyaline cartilage transplantation and grafting techniques have been developed, such as autologous chondrocytes, osteochondral allografts, and mosaicplasty (i.e., autologous osteochondral transplantation).6, 7

Satisfactory results can be obtained with autologous osteochondral transplantation, especially in combined injuries associated with subchondral bone. In this technique, described by Hangody, grafting is performed by transferring 1 or more cylindrical osteochondral autografts from low weight-bearing area of the knee to defective weight-bearing site. Along with alignment of the affected lower extremity, age of the patient, and size and location of defect, filling rate of defected area also plays an important role in long-term outcomes. This is a technically demanding procedure; matching size and radius of curvature of cartilage defects can be difficult, and some problems may arise due to complications, such as creating potential dead space between grafts and insufficient covering of defected area.10, 11, 12 Several technical or surgical problems have been pointed out in the literature regarding clinical and experimental use of autologous osteochondral transplantation with cylindrical grafts, including poor or inadequate graft fixation and graft movement (suggesting poor fit), depending on graft diameter and dilation, as well as multiple grafting, joint instability, donor host-graft size mismatch, and contour incongruence.

To overcome such complications, hexagonal grafts (honeycomb model) can be used instead of standard cylindrical grafts. Aim of the present study was to evaluate and compare hexagonal grafts and standard cylindrical grafts in terms of graft stability and capacity to fill potential dead spaces. Hypothesis of the study was that use of hexagonal grafts in mosaicplasty would provide enhanced graft stability as result of having best occupying area and volume geometry characteristics in nature.

Patients and methods

Experiments were performed using standard cylindrical osteochondral autograft transfer system and hexagonal graft system on 24 fresh, left distal femora with both condyles obtained from calves weighing between 200 kg and 400 each. None of the samples had macroscopic cartilage injury, and soft tissues were cleaned. Samples were divided into 3 groups, each consisting of 8 distal femora (Fig. 1). Hexagonal graft receivers and tunnel-opening tubes were prepared. For each cylindrical graft, 8-mm-diameter cylindrical tunnel was prepared using tunnel-opening tubes on the medial condyle load-bearing area at desired depth as described below. Similarly, 8-mm-diameter hexagonal tunnel on lateral condyle load-bearing area was also prepared for each hexagonal graft. Grafts were harvested from peripheral and superior aspects of each femoral condyle and not from the trochlea. During hexagonal graft harvesting, hammer was used to push chisels into bone to desired depth and bone plug was detached at its base with axial turning of the chisel 45° clockwise and counter-clockwise. No bony bridge was maintained between grafts. Cylindrical grafts were harvested with 90° axial turns in both directions. Graft was then inserted into recipient defect directly from donor chisel. The 3 study groups used to compare results were as follows:

Fig. 1

Samples were divided into 3 groups, each consisting of 8 distal femurs. Group 1: Single 15-mm-deep hexagonal vs cylindrical graft; Group 2: Three 15-mm-deep hexagonal vs three cylindrical grafts; Group 3: Single 5-mm-deep hexagonal vs single 20-mm-deep hexagonal graft.

First group: One cylindrical graft from lateral anterior articular surface, and 1 hexagonal graft from medial anterior articular surface of the trochlea were harvested, each 15 mm deep and 9 mm in diameter. Cylindrical graft was transplanted to medial condyle load-bearing area, and hexagonal graft was applied to lateral condyle load-bearing area in each femur (Fig. 2).

Fig. 2

Application of single hexagon graft.

Second group: Triple cylindrical grafts from lateral anterior articular surface and triple hexagonal grafts from medial anterior articular surface of the trochlea were harvested, each 15 mm deep and 9 mm in diameter. Cylindrical grafts were transplanted to medial condyle load-bearing area and hexagonal grafts were transplanted to lateral condyle load-bearing area in each femur (Fig. 3, Fig. 4).

Fig. 3

Application of 3 cylindrical grafts.

Fig. 4

Application of 3 hexagonal grafts.

Third group: A 5-mm-deep hexagonal graft from lateral anterior articular surface and 20-mm-deep hexagonal graft from medial anterior articular surface of trochlea were harvested, each 9 mm in diameter. Hexagonal graft from 5-mm-deep site was implanted to medial condyle load-bearing area and hexagonal graft from 20-mm site was applied to lateral condyle load-bearing area in each femur.

Grafts in each group were harvested from non-weight-bearing area of lateral anterior articular surface and medial anterior articular surface of the trochlea. Nine-mm-diameter cylindrical grafts were transplanted press fit to 8-mm cylindrical tunnels. Similarly, 9-mm-diameter hexagonal grafts were applied press fit to 8-mm hexagonal tunnels.

After application of the grafts, screw hooks of 5 mm in length and 2 mm in diameter were manually anchored in center of grafts after creating entry point with 2-mm Kirschner wire. Hooks were then subjected to pull-out test at rate of 20 mm/min using computer-assisted Shimadzu Autograph AGS-X 10 Kn universal testing machine (Shimadzu Corp., Kyoto, Japan) (Fig. 5). Three evaluations were conducted:

Fig. 5

Pull-out test was used to determine the load (N)–displacement (mm) curves at rate of 20 mm/min with computer-assisted Shimadzu Autograph AGS-X 10 Kn universal testing machine (Shimadzu Corp., Kyoto, Japan).

Pull-out data of cylindrical and hexagonal grafts in the 3 groups were compared. In the second group, test was conducted for only 1 of the 3 grafts. Additionally, pull-out data of 15-mm-deep single cylindrical graft in first group and 5-mm-deep single hexagonal graft in third group were compared.

Cylindrical and hexagonal grafts in first and second groups were compared in terms of friction field, which was obtained with geometrical calculation of lateral and base area of each geometric shape. Influence of friction field on stability was evaluated.

For cylindrical grafts, calculation used was:

Height (h): 15 mm, diameter (d): 8 mm

Lateral surface area: 2πrh: 376.99 mm2

Surface (or base) area: 2πr2: 50.27 mm2

Lateral surface area + base area (total friction surface): 2πrh + 2πr2: 376.99 + 50.27: 427.26 mm2

Volume: base area × h: 753.98 mm3

And for hexagonal grafts:

Height (h): 15 mm, length of 1 edge (a): 4.618

Lateral area: 6ah: 415.692 mm2

Surface (or base) area: 6 (ah/2): 55.425 mm2

Lateral area + base area (total friction surface): 6ah + 6 (ah/2): 471.117 mm2

Volume: base area × h: 831.375 mm3

Graft surface area (cartilage surface) of cylindrical and hexagonal grafts in first and second groups was evaluated by calculating base area and volume of each geometrical shape as described. Coverage ratio of defected area was assessed by comparing volume of each graft to surface area.

Statistical analysis

Mann–Whitney U test was used to evaluate significance of differences between groups using SPSS statistical software (version 15.0; IBM Corp., Armonk, NY, USA).

Results

Pull-out strength comparison between 15-mm-deep cylindrical graft and 15-mm-deep hexagonal graft in the second group revealed significant difference in favor of triple hexagonal grafts (p < 0.05). Differences between first and third groups were statistically insignificant (p > 0.05) (Table 1).

Table 1

Results of statistical analysis.

Groups	n	Main force	Standard deviation	Min. force	Max. force	P
Group 1	8	111,058 N	103,114 N	37,81 N	384,38 N	0,172
Group 2	8	92,933 N	63,082 N	27,03 N	219,06 N	0,027
Group 3	8	98,409 N	58,409 N	40 N	274,38 N	0,345

In the first and second groups, surface area (cartilage surface) and volume ratio of cylindrical and hexagonal grafts were calculated to evaluate coverage of defected space. Surface area of 15-mm cylindrical graft was 50.27 mm2, while 15-mm hexagonal graft surface area was calculated to be 55.425 mm2. Respectively, volume ratio of cylindrical and hexagonal grafts was 753.98 mm3 and 831.375 mm3 (Table 2).

Table 2

Surface area and volume ratio of cylindrical and hexagonal grafts.

Parameters	Hexagonal	Cylindrical	Difference
Surface area	55,425 mm2	50,27 mm2	5155 mm2
Lateral + base area (total friction area)	471,117 mm2	427,26 mm2	44,857 mm2
Volume	831,375 mm3	753,98 mm3	77,395 mm3

Discussion

In the present study, multiple hexagonal grafts were found to be stable as result of ability to cover larger defects without dead space between grafts. Our results demonstrated that hexagonal grafts are biomechanically superior to cylindrical grafts, particularly in multiple applications. Pull-out strength comparison between 15-mm triple cylindrical graft and 15-mm triple hexagonal grafts was statistically significantly different (p < 0.05).

Various studies have reported factors that affect stability of graft, including graft harvesting technique, graft diameter, graft length, graft tunnel length, dilation of the graft tunnel, number of applied grafts, overlapping technique and resulting layers, bone quality, and surgical technique.13, 15, 16, 17, 18

The most important factor enabling survival and persistence of harvested hyaline cartilage is transfer of graft with solid tidemark and subchondral bone. Residual dead space between grafts is filled with matrix stream, and fibrous cartilage is formed. Our study indicated that this problem may be overcome with use of hexagonal grafts, which enabled perfect filling of defected area from center to periphery. Only 1 area may remain uncovered on periphery of the defect, depending on defect configuration. In the present experimental study, mosaicplasty with hexagonal graft provided better coverage of defected area.

It is recommended that spongious bony bridge of 1 mm–2 mm in thickness be left between graft holes in mosaicplasty. This spongious bone tissue between cylindrical grafts maintains press-fit application of grafts, provides increased union, and enhances stability, especially in the short-term.20, 21 It is important to pay attention to this spongious bridge for stable fixation of cylindrical grafts; however, it is not always possible to obtain this bony bridge with optimum thickness, as it is a technically demanding surgical procedure. During operation, graft tunnels may intersect with subchondral bone, which may lead to failure of grafts. On the other hand, contiguous positioning of hexagonal grafts provides full contact on sides of grafts without the need for a wall. We hypothesized that this ease of implementation would be another advantage over cylindrical grafts, and results confirmed that stability was achieved with hexagonal grafts without necessity of spongious bony bridge between grafts.

Large defects are another issue encountered in mosaicplasty. Treatment outcomes are usually ineffective for defects larger than 6 cm2. Among the difficulties of harvesting graft of larger dimensions are fixation problems of numerous cylindrical grafts required to cover large areas, an important factor leading to unsuccessful treatment results with increased donor site morbidity and unstable graft configurations.

Graft harvesting technique is also accepted as an important factor in graft stability. Duchow et al. emphasized that graft harvesting with rotational movements is more appropriate than moving the graft side to side with levering effect, which significantly decreased graft stability. In our study, grafts were harvested with 45° rotation of hexagonal tubes. Grafts held more tightly in hexagonal tubes due to angled tube geometry and there was less need for rotational maneuvers, which aided harvesting process. Using minimal and only rotational movements as described in Duchow's study can be interpreted as a contribution to primary graft stability.

In standard mosaicplasty procedure, 60%–70% of defected area is filled with hyaline cartilage, and remaining 30%–40% with fibrous cartilage. In order to increase area filled with hyaline cartilage, it is necessary to either use smaller diameter grafts or overlap larger grafts, and each method has limitations. Smaller grafts may lead to instability and collapse of grafts, whereas overlapping may decrease fixation strength. In either case, as a result, early weight-bearing and active motion would be avoided.15, 23 Conversely, in our study we managed to cover large defected area without the need to overlap larger grafts or harvest smaller grafts to completely cover the defect area. As hexagonal grafts can be press-fit in direct contact with each other, there is no potential dead space between the grafts. Surface area and volume of cylindrical and hexagonal grafts was calculated in this study and defect coverage ratios were evaluated. Surface area for cylindrical and hexagonal grafts was determined to be 50.27 mm2 and 55.425 mm2, respectively. Volume of cylindrical and hexagonal grafts was calculated to be 753.98 mm3 and 831.375 mm3, respectively. According to our results, 10%–15% larger area may be covered with hexagonal grafts, and since there is no gap between the grafts, coverage of an even larger area may be possible.

It was calculated that there was a difference in friction field, surface area, and volume of the grafts. It was demonstrated that hexagonal grafts had greater friction field and that they could also cover a larger surface area.

As far as literature regarding osteochondral grafts of different shapes, there is only 1 patent application for cannulas with different cross-sections, including square, hexagonal, and slice-of-pie-shape. To the best of our knowledge, there is no biomechanical or clinical evaluation of these different shaped grafts, and we conducted a biomechanical study to compare the pull-out strength of cylindrical and hexagonal grafts. As this study is initial evaluation of hexagonal grafts, our preliminary biomechanical study has some major limitations. We did not plan to have any histological examination to verify percentage of transferred hyaline cartilage. Also, our experimental set-up could be modified to a loading test (push-in test), which would have greater clinical relevance, since pull-out forces are not encountered in daily life. However, push-in is technically more difficult to perform, since it requires sensors under each graft and the applied force could be absorbed by the cartilage due to indentation of the probe, so we elected to perform pull-out test to evaluate stability since friction force would be the same in either direction. Another important limitation of this study is lack of another method to evaluate area and volume of harvested grafts in order to compare and confirm values we calculated. Mathematical calculations may not be suitable or sufficient to demonstrate actual graft area and volume because harvesting and implantation process may damage the graft. Grafts are not sharp-edged objects, and as pointed in the literature, graft harvesting may damage the hyaline cartilage, especially peripheral area. Another limitation of our study was design of regular osteochondral defects rather than randomly presented defects with irregular shapes. Transarthroscopic application of hexagonal grafts has not been performed yet, and possible difficulties, such as debridement of defects in order not to have dead spaces after grafting, might be encountered during the procedure. Hexagonal graft system and operation set could be developed to include different sized tubes for irregular defects in order to overcome this problem.

Conclusion

In conclusion, based on biomechanical results, it can be stated that graft geometry, especially in multiple graft applications (according to pull-out test), is a factor that influences stability. Even though we did not find a significant difference between grafts in single application, hexagonal grafts seem to be more stable than cylindrical grafts when 3 grafts were implanted together. Furthermore, hexagonal grafts can cover larger defected area. Another advantage of hexagonal grafts is that they can be applied at shallower depth than cylindrical grafts.

Increasing stability of the graft should reduce complications, but we need additional animal and clinical studies to evaluate the relationship between early recovery and improvement of functional outcomes. However, biomechanical results of this study demonstrate that hexagonal graft system can be used in mosaicplasty.