Refixation of osteochondral fractures by ultrasound-activated, resorbable pins

Replacement therapy for osteochondral fractures

Pridie drilling or microfracture are common methods of treatment.^5,6 However, the quality and quantity of the resultant developing tissue is unpredictable with either option. Normally a layer of fibrous tissue will cover the defect almost completely.^7,8 Osteochondral grafts such as osteoarticular transfer system (OATS) and mosaicplasty can also be used,⁹ although these options risk donor-site morbidity. If the graft is not positioned precisely, chondrocyte death and fibrous tissue coverage of the gaps may occur and lead to pain and decreased range of movement.^10,11 The autologous matrix-induced chondrogenesis (AMIC) procedure shows promising results for full thickness defects, but long-term results are still not yet available.¹²

Refixation

All existing refixation systems for chondral defects have specific disadvantages, and a gold-standard has not be defined.⁴ Non-resorbable implants include Kirschner (K)-wires, Smillie pins and cortical nails.^4,13 The use of these implants can impair the quality of MRI investigations, and also necessitate further surgery in order to remove the implant. Other potential problems include loosening of the implant and the unresolved problem of rising metal ion levels in the peripheral blood.^14,15 Metal screws can ensure better and more stable fixation of the fragment, but damage to the joint surface has been observed during removal of the implant.^14,16,17

The use of resorbable implants would avoid some of these problems. More than 40 resorbable polymers have had their use described in orthopaedic implants. Polyglycolide (PGA), poly-L-lactide (PLLA), poly-D,L-lactide (PDLLA) and polydioxanone (PDA) are often used. By hydrolytic metabolism, these implants can be broken down into water and CO² in the citric acid cycle.^14,18,19 They do not require further surgery for their removal, including further morbidity or damage that may arise. However, resorbable implants often lack strong primary anchoring and long-term fixation.²⁰ For example, stability against shear forces in PLLA pins has been seen to decrease by 25% to 50% after four weeks in situ.²¹ The Ethipin (Ethicon, Norderstedt, Germany) has been used as a resorbable implant for refixation of osteochondral fragments. The handling of the Ethipin was easy, the resorption was good, but the stability was only moderate.^22-24

The ideal resorbable implant for the refixation of osteochondral fragments should have the mechanical characteristics of metal implants regarding compression force and stability against shear and pull-out forces. Most do not exhibit these characteristics.²⁰ In order to address the problem of reduced axial stability with resorbable pins, implants have been developed that anchor in the bone using ultrasound. The use of a comparable device has shown promising results in craniofacial surgery.²⁵

The aim of this study was to determine whether the SonicPin system (Stryker GmbH, Schönkirchen, Germany) has the potential to reliably refix osteochondral fragments. In order to evaluate this, we created osteochondral fragments in the knee joint of Merino sheep. Special attention was paid to the histological quality of the cartilage next to the implant, and to the heat caused by the application of ultrasound. In order to compare the SonicPin system with conventional methods, we also used the resorbable Ethipin system (Ethicon) and cannulated Asnis screws (Stryker GmbH). This breed of sheep have been used successfully in several investigations into cartilage.^26-28 The anatomy of the sheep knee and the low flexion position of the knees are similar to those of the human knee, as are the biomechanical forces encountered in vivo.²⁹

Implants

The SonicPin system (Stryker GmbH) comprises a polymer of poly(L-lactide-co-D,L-lactide) at a ratio of 70:30. It has a diameter of 2.2 mm and a length of either 22 mm or 26 mm. In preparation for use, the pin is connected to an ultrasound applicator. Then, an ultrasonic signal of high frequency and low amplitude is applied leading to a local welding of the pin tip, where the implant is in shear contact with bony structures. The SonicPin fills out the lacunes of the cancellous bone. Immediately after ceasing the ultrasound, the material solidifies and hardens, providing three-dimensional anchoring into the surrounding bone (Fig. 1). The ultrasound applicator can then be easily removed from the thread of the implanted pin.

Figs. 1a - 1c

Diagrams showing the use of the SonicPin system (Stryker GmbH), a) at implantation, mounted on the ultrasound applicator, b) after ultrasound and removal of the applicator, and c) showing the ‘melting’ of the pin into the cancellous bone.

The Ethipin (Ethicon) consists of polydioxanone and has a length of 40 mm and a diameter of 1.3 mm. It is cylindrical with no change of diameter. For use, the osteochondral fragments are temporarily fixated using a K-wire and the Ethipin subsequently inserted using the application shell. If the Ethipin protrudes the surface of the joint cartilage, it can be cut away using a surgical knife at cartilage level.

The Asnis cannulated screws (Stryker) used had a length of 16 mm and a diameter of 2.0 mm. This screw has a self-cutting thread and is made of titanium. Like the Ethipin, the screw is inserted with the use of a previously positioned K-wire. Insertion of the entire screw below the level of the cartilage prevents damage to the corresponding joint surface.

Operation procedure

A total of 16 Merino sheep were used in this study. They had an approximate age of 1.5 years, at which age epiphyseal gaps have closed,³⁰ and a mean weight of 55.4 kg (sd 4.7). Each hind leg of the sheep was used; therefore a total of 32 joints were operated upon. Prior biomechanical laboratory testing and a power analysis led to a distribution of 14 SonicPin applications and seven each of Ethipin and cannulated screws.

The operation was performed under combined anaesthesia with Rompun (Bayer Healthcare, Leverkusen, Germany) by intramuscular injection and spinal anaesthesia with Carbostesin (AstraZeneca, Wedel, Germany). Using medial arthrotomy, the medial condyle was exposed and a specific osteochondral fragment (1 cm diameter and 4 mm thickness) was created with a dedicated saw guide to simulate a typical osteochondral defect (International Cartilage Repair Society (ICRS) Grade 4).³¹

The fragment was then refixed using the previously randomised system of fixation. Two implants were used for each osteochondral fragment. The randomisation was done just before the refixation was performed in the operating theatre by opening opaque envelopes. Each hind joint of each animal was randomised, so different implants in the two hind knees of one animal could occur. The drilling process and the melting of the SonicPin were monitored by a FLIR ThermaCam E320 heat camera (FLIR Systems Inc., Wilsonville, Oregon). All implants used were implanted with a drill guide to ensure that the two implants were at an angle of 15° to each other. A soft cast and a sterile bandage were applied after the arthrotomy was completed. The soft cast was removed after five days. Full weightbearing was intended for the entire duration of the study for worst-case scenario testing.

At three months post-operatively the sheep were killed and the knees were removed. After opening the knees, the ratio of well-healed fragments were observed in addition to macroscopic scoring of the fragments and the corresponding cartilage surface by Outerbridge score (Table I).^32,33 We defined a refixed fragment as ‘healed’ if it was in position and at least > 50% present.

Histological analysis

For histological assessment, the femoral probes only were fixated in 4% paraformaldehyde and decalcified by ethylenediaminetetraacetic acid (EDTA) over about eight weeks, because especially the interaction of the cartilage with the implant should be examined. For microscopic examination, haematoxylin & eosin and Giemsa staining was performed. The thickness of the cartilage, the clusters of chondrocytes found, the amount of fibrous tissue scars and the number of chondrocytes in each chondron were observed microscopically. In order to do this, a grid was projected behind the microscopic sample and the ten areas next to the implant and ten areas in the clinically unaffected cartilage area were counted and compared with each other. The amount of fibrous tissue and clustering of chondrons were also observed in this way. The orientation of the chondrons and the amount of fibrous tissue were both rated by two authors (HN and BK) on a scale of 1 to 5 points, with 5 points denoting no fibrous tissue and vertical orientation of the chondrones, and 1 point denoting only fibrous tissue, no cartilage left and circular clustering of chondrons (Table II). The thickness of the cartilage was measured by the same authors (HN and BK) in millimetres from the implant (macroscopically unaffected surface) and next to the implant by measuring the height from the surface to the subchondral plate.

Statistical analysis

The statistical analyses were performed using the t-test and the Levene test. SPSS software v20 (IBM Germany, Ehningen, Germany) was used, and the level of significance was assumed to be p < 0.05.

Temperature

The temperature data were available for all implants, including those in animals who died prematurely. The mean temperature observed during the drilling of the Ethipin system was 46.83°C (sd 1.47), significantly lower than that reached using during drilling of either the SonicPin (54.14°C (sd 2.41); p < 0.001) or the Asnis screw (54.88° (sd 2.94); p < 0.001). There was no statistically significant differences between the temperatures reached during drilling of the SonicPin and Asnis screw systems (p = 0.12). The mean temperature reached by the melting of the SonicPin implants was 50.57°C (sd 2.0), which was significantly lower compared with the drilling temperatures of the Asnis screw and SonicPin (p = 0.0002 and 0.0006, respectively), and significantly higher compared with the Ethipin drilling temperature (p = 0.0007).

Healing ratio

We defined a refixed fragment as properly healed if it was in position with > 50% of the original surface present. By these criteria, healing was seen to occur in nine of the 14 fractures treated using the SonicPin system, compared with three of six treated using the Ethipin and seven of eight treated with the Asnis cannulated screws, giving healing ratios of 64%, 50% and 87.5% for the SonicPin, Ethipin and Asnis screws, respectively. The Asnis screws performed significantly better than the SonicPin (p = 0.045) and the Ethipin (p = 0.23), although there was no significant difference between the healing ratios of the two resorbable implants (p = 0.18).

We observed that the failures in the two resorbable implant groups often exhibited a shearing mechanism in the original osteotomy gap with broken pins. Conversely, the single failure in the screw group showed the implants in their original position, while the fragment was lytic around the osteosynthesis.

Macroscopic scoring

Macroscopic scoring of the chondral surfaces of the refixated fragment and the corresponding tibial plateau was performed according to the Outerbridge classification (Table I, Figs 2 and 3).^32,33 Cases in which the fragment sheared away into the former osteotomy gap were rated as grade 4.

Figs. 2a - 2b

Photograph at macroscopic assessment showing the SonicPin (Stryker GmbH) fixation a) in the femoral surface and b) the corresponding tibial surface, showing minimal damage.

Figs. 3a - 3b

Photograph at macroscopic assessment showing the Asnis screw (Stryker GmbH) fixation a) in the femoral surface and b) the corresponding tibial surface, showing marked and deep chondral damage.

Table III details the mean Outerbridge scores for the femora and tibiae for each group. Although there were no significant differences between the implants on either the femoral or tibial side, the majority of the documented tibial defects were markedly deep.

Histological scoring

The mean measured thickness of the femoral cartilage next to the implant in the SonicPin group was 88% (sd 19) of the original height of the cartilage. The mean thickness was also 88% (sd 25) in the Asnis group, and 74% (sd 32) in the Ethipin group. These results did not differ significantly (SonicPin vs Ethipin, p = 0.205; SonicPin vs Asnis, p = 0.314; Asnis vs Ethipin, p = 0.115).

The cell count in each chondron near and distant from the implant showed a higher number of cells nearer the implant in all cases (Table IV). This difference was statistically significant in the SonicPin group (p = 0.006), but not in either the Ethipin or Asnis groups (p = 0.184 and p = 0.179, respectively).

The best vertical orientation of all implants was observed in the screw group, compared with both both resorbable implants (p = 0.143 and p = 0.253 for SonicPin and Ethipin, respectively) (Fig. 4, Table V). Similarly, the resorbable implants exhibited a little more fibrous tissue reaction in comparison with the screw fixation, but these did not reach statistical significance.

Figs. 4a - 4b

Histological images of cartilage after removal of the SonicPin system (Stryker GmbH), a) at × 25 magnification, showing the cartilage surface without major damage after removing the SonicPin, and b) at × 40 magnification, showing the cartilage next to the removed SonicPin.

Conclusions

In a sheep study, the SonicPin system has been successfully used to for the refixation of osteochondral fragments. The healing ratio is higher than with the Ethipin system. A negative effect on the cartilage by the heat generated by the SonicPin system cannot be proven. No further surgery is necessary for removal of the implant, and MRI imaging is not compromised.