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The TMT-1 joint arthrodesis is a common repair for severe hallux valgus. Two crossing interfragmental screws, usually titanium or steel, and a locking plate or a plate with a compression screw are the most common fixation methods for first TMT joint arthrodesis. The qualities of an ideal fixation material include adequate strength and rigidity, biocompatibility, lack of interference with bone healing, lack of visibility and palpability, and a low risk of surgical removal. We sought to determine whether bioabsorbable cannulated screws would perform as well as titanium screws in anatomical models.
Methods
Identical anatomical TMT-1 arthrodesis was created with a saw by making a straight cut in 30 anatomical models (Sawbone®). The bioabsorbable and titanium screws were placed one at a time in exactly the same location in each model according to careful measurements. All 30 models were analyzed with a material testing machine (MTS Insight 30, Eden Prairie, USA). Each model was oriented 15° to the platform to simulate its position to the ground during mid-stance.
Results
In the single-cycle load-to-failure test, the mean yield load was 61.4 N ± 5.7 N (range, 50.1 N–70.3 N) in the bioabsorbable screw group and 81.2 N ± 12 N (range, 61.7 N–113.4 N) in the titanium screw group (P < .001). The respective values for the stiffness of the fixation were 8.1 N/mm ± 0.8 N/mm (range, 6.7 N/mm to 9.1 N/mm) and 9.7 N/mm ± 1.8 N/mm (range, 6.9 N/mm to 12.6 N/mm) for the bioabsorbable and titanium groups (P = .004). The mean maximum failure loads in the bioabsorbable group were 85.1 N ± 8.5 N (range, 67.1 N–97.2 N) and in the titanium group 120.6 N ± 13.2 N (range, 96.7 N–136.7 N), respectively (P < .001). Analysis of the failure models shows bioabsorbable fixation failures caused by bending occur more often than in the titanium group.
Conclusion
In biomechanical testing, titanium screws were stronger than bioabsorbable screws in the TMT-1 arthrodesis model tested, although bioabsorbable cannulated screws may be an alternative to titanium screws in the fixation Lapidus procedure.
The first tarsometatarsal (TMT-1) joint arthrodesis (the Lapidus procedure) has been a common procedure since it was first introduced by Lapidus in 1934.
The procedure is technically demanding and has been associated with a prolonged period of recovery and increased morbidity when compared with metatarsal osteotomies.
The TMT-1 joint arthrodesis provides good correction and stability to the first ray. Klemola and colleagues have suggested an operation technique for TMT-1 joint arthrodesis that accentuates the rotational correction of the first ray.
First tarsometatarsal joint derotational arthrodesis--a new operative technique for flexible hallux valgus without touching the first metatarsophalangeal joint.
The rotational stabilization of the TMT-1 joint and the improved function of the windlass mechanism and peroneus longus could enhance the weight-bearing properties of the foot and have a satisfactory long-term outcome.
Two crossing interfragmentary screws, usually titanium or steel, and a locking plate or a plate with a compression screw are the most common fixation methods used for TMT-1 joint arthrodesis. After TMT-1 joint arthrodesis, the rate of patient satisfaction varies between 75% and 90% with fusion rates of approximately 90%.
Peterson and colleagues have reported that the rate of hardware removal after TMT-1 arthrodesis was 15% of all patients. Of these, 18 patients (72%) had a locking plate and lag screw removed, and 7 patients (28%) had crossing lag screws removed.
Another study has reported hardware removal rates of 12% (an intraplate compression screw fixation), 11% (crossing solid core screw fixation), and 0% (a single interfragmentary screw with a simple locking plate).
Most bioabsorbable implants are composed of the monopolymers or copolymers of polylactic acid (including a levorotary and dextrorotary configuration), polyglycolic acid, and polydioxanon.
The rate of biodegradation depends on several factors, such as the chemical composition of the material, molecular weight, impurities, crystallinity, sterilization, shape and sizes of the implant, and surface quality.
The main advantage of using bioabsorbable implants is the reduced need for secondary surgery.
The fixation strength of bioabsorbable implants, such as screws, remains a source of argument, and a recent study showed that metallic screws are stronger in the fixation of tibial tubercle transfer in cadavers than bioabsorbable screws. The study did suggest, however, that the fixation strength of bioabsorbable screws seemed clinically sufficient.
The qualities of an ideal bioabsorbable fixation material include adequate strength and rigidity, biocompatibility, lack of interference with bone healing, lack of visibility and palpability, and a low risk of surgical removal. Bioabsorbable implants are well-known alternatives for the titanium implants used in the internal fixation of certain fractures and osteotomies, such as TMT-1 joint arthrodesis, radial head fractures, ankle fractures, Chevron osteotomy, and proximal tibiae osteotomy and fracture.
Arthrodesis of the first metatarsophalangeal joint in patients with rheumatoid arthritis with bioabsorbable self-reinforced poly(L/DL)lactide 70:30 screw fixation- a preliminary report.
In this study, we sought to determine whether bioabsorbable cannulated screws could perform as well as titanium cannulated screws in an in vitro study of the foot with a simulated hallux valgus deformity corrected with TMT-1 joint arthrodesis using two crossing interfragmentary lag screws for fixation. To the best of our knowledge, there have been no previous studies that have evaluated bioabsorbable materials in TMT-1 joint arthrodesis or compared them with titanium.
2. Methods
This study compared TMT-1 joint arthrodesis divided into two groups each comprising 15 anatomical models (Sawbone® anatomical foot models, Sawbones Europe AB, Malmo, Sweden) using two crossing interfragmentary lag screws. These 15 anatomical models were chosen to reduce the variability introduced by the use of cadaveric specimens. The bioabsorbable screws were manufactured from poly(l-lactide-co-glycolide) (85L/15G) raw material (resomer LG857, Ingelheim am Rhein, Germany). In this study, we used the ActivaScrew™ Cannulated LAG 3.5 mm✕35 mm and 3.5 mm✕40 mm screws and the Stryker Asnis™ Cannulated Screw 4.0 mm✕36 mm and 4.0 mm✕40 mm. Although the manufacturers claim there are differences in the diameters of the screws, the real dimensions of the screws are very similar (Fig. 1). The dimensions of the bioabsorbable screw were head 5 mm, body 3 mm, and thread 3.5 mm, and the dimensions of the titanium screw were head 5 mm, body 2.5 mm, and thread 4 mm.
Fig. 1Bioabsorbable (left side) and titanium screws (right side) used for fixation in performing. the Lapidus procedure to treat severe hallux valgus.
Each anatomical foot model was modified by cutting the TMT-1 joint using a saw and making a straight cut between the medial cuneiform and the first metatarsal bone. The bioabsorbable and titanium screws were then placed one at a time in exactly the same location in each model by the same researcher. Measurements were taken using a drawing device to determine the entry and output points for Kirschner wires (K-wires) (Fig. 2, Fig. 3). The Lapidus fusion was then temporarily fixed with the K-wires. The guide wire was then overdrilled with a 2.7 mm cannulated drill for the bioabsorbable screws and a 3.5 mm cannulated drill for the titanium screws. The channels were tapped without fully closing the osteotomy site. The cannulated cortical lag screw was inserted from dorsal-to-plantar over the osteotomy using K-wires. The longer bioabsorbable (3.5 mm✕40 mm) and titanium (4.0 mm✕40 mm) screws were inserted into the first metatarsal to the medial cuneiform, and the shorter bioabsorbable (3.5 mm✕35 mm) and titanium (4.0 mm✕36 mm) screws into the medial cuneiform to the first metatarsal. The K-wires were then removed.
Fig. 2The Sawbones anatomical model of severe hallux valgus from above.
All 30 models were analyzed using a material testing device (MTS Insight 30, Eden Prairie, USA). The biomechanical testing protocol consisted of the single-cycle load-to-failure test. In the single-cycle load-to-failure test, each model was oriented 15° to the platform to simulate its position to the ground during mid-stance (Fig. 4). Then, the models were subjected to repetitive plantar-to-dorsal loading at a compression rate of 5 mm/s. The response of each model to the loading was automatically recorded as a load-displacement curve, and the stiffness (determined as the slope of the linear region of the load-displacement curve corresponding to the steepest straight-line tangent to the loading curve), yield load (described as the load at the point where the slope of the load-displacement curve first clearly decreased), and maximum failure load were determined.
All the tests were video-recorded, and the failure mechanism was analyzed afterwards.
Fig. 4The Sawbones anatomical model positioned in the material testing machine. Compression. was applied at 15° to the platform and at a rate of 5 mm/s.
Data are shown as mean ± SD unless otherwise stated. The mean values of the variables were compared between groups using the paired t-test. Alpha was set at 0.05. The data were analyzed with SPSS Statistics 24 (IBM).
3. Results
In the single-cycle load-to-failure test, the mean yield load was 61.4 N ± 5.7 N (range, 50.1 N–70.3 N) in the bioabsorbable screw group and 81.2 N ± 12 N (range, 61.7 N–113.4 N) in the titanium screw group (P < .001). The respective values for the stiffness of the fixation were 8.1 N/mm ± 0.8 N/mm (range, 6.7 N/mm to 9.1 N/mm) and 9.7 N/mm ± 1.8 N/mm (range, 6.9 N/mm to 12.6 N/mm) for the bioabsorbable and titanium groups (P = .004). The mean maximum failure loads in the bioabsorbable group were 85.1 N ± 8.5 N (range, 67.1 N–97.2 N) and in the titanium group 120.6 N ± 13.2 N (range, 96.7 N–136.7 N), respectively (P < .001) (Table 1).
Table 1Fixation measurements for titanium and bioabsorbable screws.
The bioabsorbable fixation showed deformation of 9.0 mm ± 1.4 mm (range, 7.7 mm–11.8 mm) compared with titanium fixation of 9.9 mm ± 2.2 mm (range, 7.4 mm–16.1 mm) (p = .271). The failure modes of the bioabsorbable screws and the titanium screws are presented in Table 1. An analysis of the failure models showed a difference in breakage models between the bioabsorbable and the titanium groups. The titanium screws failed (6/15) as the result of bending. In other cases, failure occurred as the result of the breakage of the cuneiform medial caused by a small split (9/15). In the bioabsorbable group, breakage occurred more often due to (11/15) bending. One bioabsorbable model failed by the pull out of the metatarsal at the site of screw (1/15), and two bioabsorbable models broke the cuneiform medial (2/15), as was the case with most of the titanium screw failures (Table 2).
Table 2Fixation failure modes of titanium and bioabsorbable screws.
To the best of our knowledge, this is the first study to compare bioabsorbable and titanium screw fixation in TMT-1 arthrodesis. The results of the study show that titanium screws are stronger than bioabsorbable screws in TMT-1 arthrodesis. However, although titanium screw provided higher fixation strength, the bioabsorbable screws also exceeded the estimated maximum force required to retain stability until ossification occurs in first metatarsal bone.
We consider that both screws could provide clinically sufficient strength for TMT-1 arthrodesis fixation.
Bioabsorbable fixation implants have several clinical advantages when compared with titanium implants. The most important of these is that the use of bioabsorbable implants eliminates the need for secondary surgery due to implant removal. Peterson and colleagues showed that incidence of symptomatic hardware removal after TMT-1 arthrodesis is approximately 15% and commonly occurs 9 months after surgery.
Furthermore, in contrast to titanium implants, bioabsorbable implants do not interfere with imaging or cause stress shielding, growth restriction, or an accumulation of titanium in tissues.
Bioabsorbable screws are more elastic and flexible than titanium screws, and the splits near the insertion points noted in the titanium screws occurred less often with the bioabsorbable screws (Fig. 5). This difference can be explained by the different composition of the screws and the interaction with the Sawbones® material. This Sawbone® study setup cannot correlate strictly with the clinical aspect, but the different stress holding between the titanium and bioabsorbable screw groups indicates that bioabsorbable material could be a better choice with regard to the stress holding aspect. This bioabsorbable screw characteristic could lead to reduced stress shielding, while incrementally transferring load to healing bone.
Fig. 5Breakage caused by small split with titanium screw fixation.
Several studies have reported good results in using bioabsorbable materials in foot and ankle surgery. Kim and colleagues reported that the use of bioabsorbable screws in scarf osteotomy yielded good radiographic outcomes and good patient satisfaction with only a small number of complications.
Bioabsorbable fixation with a bioabsorbable pin has been used in Chevron osteotomy with good outcomes, and one study showed that fixiation with a bioabsorbable pin was as reliable as fixation with a metal screw and allowed major angular corrections in 5-year follow up.
The bioabsorbable screw fixations were comparable to other fixation devices for PIP fusion fixation regarding success rate, revisions, and patient acceptance.
Bioabsorbable screws have also been used in foot and ankle trauma. Ahmad and Jones showed that bioabsorbable screws are comparable and not significantly different from steel screws for treating unstable Lisfranc injuries.
The use of bioabsorbable screws have also been reported in calcaneal fractures, and they provided sufficient stabilization and calcaneal fracture healing/union with the added advantage of no implant removal needed.
It seems that there are good results in a variety of indications for the use of bioabsorbable implants, although several studies have shown that titanium or steel screws are stronger than bioabsorbable screws.
The main disadvantage of bioabsorbable implants is the difficulty in controlling the degradation process. The rate of biodegradation depends on several factors, such as the chemical composition of the material, its molecular weight, the presence of impurities, its crystallinity, the sterilization process, the shape and size of the implant, and its surface quality.
this quality offers temporary mechanical support for the bone until it gradually gains its original load-carrying capacity.
We selected the ActivaScrew™ cannulated lag screw and the Stryker Asnis™ cannulated screw for testing because the profiles of the screws are similar (Fig. 1). Still, the screws differed in that the titanium screws had a wider thread than that of the bioabsorbable screws. This wider diameter provides stronger fixation and it could help explain the differences between the two groups. One bioabsorbable screw fixation failure caused by pull-out could be explained by the thinner thread diameter of the bioabsorbable screw. This finding indicates that screw geometry could be of remarkable significance for the biomechanical properties of bioabsorbable screws.
Our study has some limitations. Although biomechanical testing with Sawbones® products is widely used, and thus an acceptable method of testing different fixations,
biomechanical results using this material are not directly clinically comparable, since synthetic bones may not accurately reflect the properties of real bone. Further, the degradation process of bioabsorbable screws and changes in mechanical properties during the degradation process cannot be tested with Sawbones® models. On the other hand, the advantages of synthetic bones are the homogeneity of specimens and the good reproducibility of fixation.
Bioabsorbable cannulated screws are a potential alternative to titanium or metallic screws for TMT-1 arthrodesis. The cannulated screws make fixation easier by using Kirschner wires, and bioabsorbable screws avoid the possibility of a second surgery to remove the screws. Further testing of these screws in larger and adequately powered clinical studies is feasible.
Financial disclosure
“None reported”.
Declaration of conflicting interest
‘The Authors declares that there is no conflict of interest'.
Acknowledgements
‘There are no acknowledgements'
References
Robinson A.H.
Limbers J.P.
Modern concepts in the treatment of hallux valgus.
First tarsometatarsal joint derotational arthrodesis--a new operative technique for flexible hallux valgus without touching the first metatarsophalangeal joint.
Arthrodesis of the first metatarsophalangeal joint in patients with rheumatoid arthritis with bioabsorbable self-reinforced poly(L/DL)lactide 70:30 screw fixation- a preliminary report.
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