USE OF BIOABSORBABLE IMPLANTS FOR HALLUX VALGUS SURGERY.
Central State Medical Academy, Moscow, Russia
Hallux (abducto) valgus is the most common term in the literature. It designates any deformations at the level of the medial metatarsophalangeal articulation with valgus declination of the toe in most cases [1].
There is an interesting fact about absence of clear absolute signs for precise determination of the border between the normal value and the valgus declination of the toe, but the values above 15 degrees for the first metatarsophalangeal articulation (hallux valgus angle, HVA) or more than 9 degrees for the first intermetatarsal angle are considered as pathologic. However there are individuals with higher values of the first metatarsophalangeal and intermetatarsal angles, but without symptoms of hallux valgus [2].
Due to absence of any clear criteria determining the disease, it is quite difficult to estimate the incidence of the pathology in the population. According to some authors, it varies from 19 to 70 % [1-3].
The difficulty of pathogenesis of development of the disease requires the differential approach and estimation of direction of medical measures. Some priority directions for treating his pathology have appeared. They can be divided into conservative and surgical, but there is not any uniform opinion about features and volume of radical treatment [3].
Conservative treatment is based on slowing down progression of the pathologic process in the foot and is the method of choice for patients who request medical care for hallux valgus for the first time [4]. It is oriented to decreasing non-fixed valgus deformation, expansion of wrinkled soft tissues around the joint with use of night time splints, and increasing muscular tone of the foot with use of exercises, as well as wearing the orthopedic soles [5]. Juriansz (1996) conducted the randomized study including the patients with night splinting and the patients without treatment. There was not any significant difference in valgus declination, the value of the first intermetatarsal angle and pain intensity between the groups [6]. A similar randomized study of functional orthoses was conducted by Kilmartin et al. (1994). It included 122 children (age of 9-10). Three years later, the examination identified a statistically significant difference in the angle of valgus deformation in the study group [7]. Most researchers concluded that the conservative techniques were inefficient for adults [1, 2, 8].
Since the ancient times the surgeons paid their attention to the pathology of the first metatarsophalangeal articulation. In 1267 Theodorice wrote: “firstly, it is necessary to remove everything around and then to cauterize the spur”. In 1862 Boyer recommended ablation of a cyst in the first metatarsophalangeal articulation. In 1873 Fricke described two cases with round exostoses on the foot. He resected the bones forming the first metatarsophalangeal articulation and received the excellent results. The results of resection of the first metatarsophalangeal articulation were published by Pancoast in 1844 and Hilton in 1853. In 1874 Rose removed the sesamoid bones as a part of joint resection. Reverdin (1881) advocated only removal of exostoses. In 1904 Keller initiated resections of the basis of the proximal phalanx of the toe [1, 8].
In the 20th century the number of operations for correcting hallux valgus increased significantly. Metcalf (1912) summarized 15 different types of surgery, Timmer (1930) described 25 types, Verbrugge (1933) – 51, Perrot (1946) – 68. For the moment of 1990 about 150 various types of surgery (Luthje, 1990) were described. At the present time there are about 400 surgical techniques for static deformations of the anterior part of the foot [9, 10]. The significant amount of surgical techniques indicates the absence of a uniform technique and presence of serious disadvantages. For achievement of good functional outcomes the choice of a surgical technique should depend on the anatomical features and the basics of pathological changes in a patient [2, 8].
For selecting a surgical technique one can use a lot of the classifications, which are based on the various principles: anatomical principle (soft tissues of bones), a type of surgical intervention, surgery localization. The selected technique must correct all elements of deformation: osteophyte of the head of the first metatarsal bone, valgus deformation of the proximal phalanx of the toe, increase in the first intermetatarsal angle, congruity of articular surfaces, subluxation of sesamoid bones and toe pronation [9].
Some types of osteotomy of the first metatarsal bone became popular. The offered techniques include diaphysis and metaphysis osteotomy (the proximal and distal ends of the first metatarsal bone). They are different according to the direction of osteotomy line [10, 11, 12].
Regardless of choice of osteotomy type, fixation is performed with various metal constructs. The analysis of the complications after osteotomy of the first metatarsal bone showed that the significant proportion of the complications was associated with metal implants. Metal implants provide proper fixation of bone fragments, but rigidity of fixation is excessive and it leads to osteolysis on the border between metal and the bone (stress-shielding syndrome) and migration of metal constructs [13, 14, 15]. According to the data from many researchers, this event is associated with diverse elasticity of bone tissue and metal (Young 's modulus – 10-30 GPa for the cortical bone and 100-200 GPa for metal). The second disadvantage of metal fixators is need for recurrent surgery after its removal [16].
Owing to the high incidence in the population, the pathology has both medical and economical meaning. Therefore, the researchers have been trying to improve the outcomes of surgical treatment, to reduce hospital stay and to decrease the costs of treatment. The surgical techniques and the methods of fixation of the first metatarsal bone after osteotomy have been improving [17].
The “ideal” fixator for osteotomy should provide the adequate stability of bone fragments, have sufficient strength up to the moment of full union and absorption after union, with excluding the necessity of recurrent surgery for removal of the fixator [18].
History of biodegradable materials
α-polyhydroxyacids is a class of synthetic ester polymers of α-hydroxyacids. The most common types of this class are polylactic and polyglycolic acids.
Bischoff and Walden synthesized polyglycolic acid (PGA) in 1893. Its high-molecular polymer with elastic properties was synthesized by Higgins in 1954. It became the first absorbing suture material. The polymers of glycolic acid are solid crystal compounds, insoluble in fluid, with the melting temperature of 224-228°C [20]. Biodegradation of complex polyesters is realized by means of non-specific hydrolytic breakdown to carbon dioxide and water [21]. The time of absorption depends on the environment conditions, molecular mass and the implant size. Mechanical strength of PGA disappears after 4-7 weeks, and full absorption of the polymer takes place in 6-12 months (from 12 weeks to 9 months according to the various authors) [22]. According to Vasenius (1990), PGA is the strength material with rigidity, which is sufficient for fixation of most fractures. However because of hygroscopicity, degradation of the polymer is too rapid [23]. Because of fast degradation and development of aseptic synuses, the implants made of “clear” PGA are not used for osteosynthesis [24].
The polymers of lactic acid (polylactides, PLA) are hemicrystalline and hydrophobic. PLA consists of recurrent links of lactic acid, with two stereoisomeric forms of L and D-isomers. L-isomer originates in the human body, for example, as result of anaerobic metabolism of glucose. The level of D-isomer in the body is extremely low. L-isomer is characterized by high mechanical strength and low absorption. Therefore, it is used for production of orthopedic implants. The high-molecular synthetic polymer of lactic acid with thermoplastic properties was created by Schneider in 1955. As compared to other PGA biodegradable implants, poly-L-lactic acid (PLLA) polymers have longer period of degradation (2-6 years) [25, 26]. Degradation of polylactide is caused by non-enzymatic hydrolysis with breakdown to pyruvate. The time of degradation depends on the ratio of polymers in the implant, solidity and molecular weight and it lasts for 9.3 years according to Voutilainen et al. (2002) [27]. The advantages and the disadvantages of each polymer resulted in development of the copolymer implants including L- and D-isomer of lactic acid. The rate of absorption and mechanical strength depend on the quantitative level of various isometric forms of L- and D-monomers in the polymer chain. The strength of PLA copolymers can be significantly increased by mixing with absorbable rubbers such as trimethylene capronate [28].
Biodegradable polymers were firstly mentioned in the literature in the end of 1960s. In 1966 Kulkarni et al. published the report about biocompatibility of L-polylactide (LPLA) in the animals. Polymer was implanted in guinea pigs and rats. The powder form was used. The researchers found that the polymer was non-toxic, without responses in surrounding tissues and with long period of degradation. In 1971 the researchers presented the animal study of biodegradable plates and screws made of the same polymer (LPLA) for fixation of fractures of the lower mandible [28]. At the same time, Cutright et al. published the similar study [29]. Both studies showed that the material did not cause inflammation or response to a foreign body, although absorption was not complete in the end of the study. In 1984 Rokkanen et al. were the first who had used biodegradable implants for fixation of the ankle join in the human (Helsinki, Finland) [30].
Currently, PLA and PGA copolymers, i.e. polylactide-glycolide (PLGA), are used. Bioabsorbable implants have some important advantages as compared to metal implants, for example, gradual increase in load to a healing bone (while degradation of polymer continues) and absence of necessity for removal of the fixator [31]. According to the analysis of Cochrane database (Jainandunsing et al., 2009), there were no statistically significant differences between biodegradable implants and other implants concerning long term results, functional status and complications [32]. The percentage of recurrent surgery is lower for use of these fixators as compared to other groups. The authors concluded that fixation of simple fractures (type A) with biodegradable implants was an appropriate technique of treatment with low rate of complications and 20 % decrease in costs [33].
Some authors state that the biomechanical properties of bioabsorbable screws and plates are similar with metal ones, if used for small tubular bones [34], others believe that bioabsorbable implants have lower mechanical strength and torsion stability as compared to metal [35] that is the advantage for fixing fractures with small fragments, fixation of arthrodesis of small joints, osteotomy of small bones and fixation of ligamentous and soft tissue structures of the humeral and knee joints.
The results of use of bioabsorbable materials for fixation of the first metatarsal bone
The first reports about use of absorbable fixators for correction of hallux valgus described the complications after use: development of granuloma around the implant, soft tissue response, loss of mechanical stability of the implant and lost correction of the first metatarsal bone [36-38]. But the problem of osteolysis was still actual, although bioabsorbable materials were initiated for elimination of this problem. Burns et al. identified the radiological signs of osteolysis in more than 22 % of operated patients, but it did not influence on the functional outcome [39].
There were reports about efficient use of biodegradable screws. Brunetti VA et al. described a single case with use of bioabsorbable screws for fixation of chevron osteotomy with satisfactory outcomes [40].
Along with search of optimal ratio of polymers and improvement in bioabsorbable implants, the researchers reported about the satisfactory results. Hirvensalo et al. produced 78 procedures of chevron osteotomy of the first metatarsal bone and fixation with PGA screws. In 75 % of the cases the results were excellent and good, but 15 % of the patients complained of pain in the first metatarsophalangeal joint during load, and in 10 % - recurrent deformation [35]. Other authors also reported about the positive results of use of biodegradable implants [41-49].
DeOrio and Ware described the results of fixation with use of polydioxanone pins and achieved the satisfactory level of correction. Moreover, there were no such complications as osteolysis, infection, aseptic necrosis of the femoral head or non-union of osteotomy [46].
Winemaker et al. compared the results of fixation with pins and bioabsorbable screws. The functional index AO FAS was similar in two groups. Pin fixation resulted in complications in 4 patients among 21, with no complications in other group [47].
Barca et Busa conducted 35 procedures of chevron osteotomy with PLA screw fixation. Stable fixation and normal postsurgical bone union were noted in all cases. 90 % of the patients reported about satisfactory functional and cosmetic results. One case was complicated by aseptic necrosis of the head of the metatarsal bone [48]. Surrounding tissue response exists in use of metal fixators and bioabsorbable ones, with manifestations in view of granuloma, sterile sinus, osteolysis and fibrous alteration of tissues around the implant.
Morandi A (2013) published the results of use of bioabsorbable screws for fixation of chevron osteotomy (439 operations for 5 years). The authors concluded that use of polymer screws resulted in good clinical and radiological outcomes (also long term results within 5 years) and the low number of complications [45].
Besides the medical aspects of bioabsorbable screws, some economical issues exist. Valletjo-Torres L et al. (2011) conducted the study of economical efficiency of bioabsorbable screws as compared to metal ones. They concluded that despite of increasing costs for production of such implants, the final costs for treating one patient decrease due to absence of necessity for recurrent surgery for removal of the fixator [50, 51].
FINAL STATEMENT
Therefore, the analysis of the foreign and domestic literature showed the absence of any uniform opinion about bioabsorbable materials for correction of hallux valgus.
Some authors consider that efficiency and reliability of polymers are similar with metal fixators with low rate of complications. Other researchers state that polymers are fragile and osteolysis develops around the implant, as well as granuloma appears.
The biomaterials have been improving during the last 50 years. In the modern biodegradable implants the percentage ratio of polymers of lactic and glycolic acids is optimal for stable fixation of the bone and degradation of polymer after osteotomy union.
The economic aspect is also discussed. From one side, production of polymers is the expensive high-tech process that influences on the final price of the implant. From other side, there is no need for removal of the implant, i.e. no recurrent admission and decreasing costs for treating one patient.
Considering the above-mentioned facts, the actual issue exists: a possibility for use of the modern bioabsorbable fixators instead of standard metal ones, the influence on the functional status and quality of life in patients with hallux valgus.
CONCLUSION
1. The modern bioabsorbable implants demonstrate the sufficient strength for fixation of bone tissue with degradation of polymer after union of osteotomy of the first metatarsal bone.
2. The characteristics of rigidity of bone tissue and biopolymers are similar. As result, stress-shielding syndrome does not develop.
3. The economic appropriateness of biodegradable implants is not estimated: the production is expensive, but there is no need for removal of the fixator.
4. The rate of complications after use of biodegradable implants and metal ones is similar for fixation of osteotomy of the first metatarsal bone.
5. There are not any studies concerning the quality of life after correcting osteotomy of the first metatarsal bone with use of biodegradable implants.