The practice of limb lengthening dates back to 1869 when Langenbeck postulated that bone growth could be stimulated by axial traction. The first recorded lengthening procedure was performed by Codivilla in 1905 and consisted of an osteotomy followed by axial traction and immobilization in a Thomas splint. The emergence of external fixators, which allowed controlled distraction together with Gavril Ilizarov's work, led to rapid advances in the practice of limb lengthening and our understanding of the biology of distraction osteogenesis.
Despite the accurate lengthening that external fixators provide, these devices are associated with high complication rates, including pin site infection, deep sepsis, neurovascular injury, joint contractures, and stiffness. Refractures after fixator removal and the discomfort and psychological stress are further drawbacks to using these devices.
To overcome these problems, internal lengthening devices have gained popularity. Alexander Bliskunov developed the first internal lengthening device in 1983. Götz and Schellmann further developed this concept with a hydraulic distractor nail, whereas Baumann and Harms later developed an intramedullary extension nail. These developments later gave rise to the development of multiple other intramedullary lengthening devices.
In 1989, Betz and Baumgart introduced a motorized lengthening device without a telescopic principle. The nail functions by an internal motor connected to a subcutaneously implanted antenna activated by external radiofrequency stimulation. In 1988, Grammont and Guichet developed the Albizzia® telescopic nail (DePuy, Villeurbanne, France), which consisted of two telescoping tubes: a threaded outer tube and an inner rod, connected by a double-opposed ratchet mechanism. By rotating the inner tube by 20°, the ratchet mechanism is unscrewed, and the nail is lengthened by 1/15 of a mm. The nail then resets when the nail is rotated back to the resting position. Guichet modified the Albizzia® nail and named it the G-Nail® (X-os S. A., Lugano, Switzerland). Betz later modified the ratchet mechanism and direction, dimension, and design of the interlocking system and developed the Betzbone® nail (Betz Institute, Wadern, Germany) and changing the orientation of the locking holes. A second-generation Betzbone nail introduced the use of cobalt-chrome as a nonferrous metal instead of surgical steel.
In 2001, the intramedullary skeletal kinetic distractor (ISKD) (ISKD, Orthofix, Verona, Italy) was released, which used the same lengthening principle by rotation of a ratchet mechanism. Contrary to the Albizzia® nail, only 3° to 9° of rotation was needed to perform lengthening. In 2009, Pool and Walker developed the Precice® (Nuvasive, San Diego, CA, USA) magnetically driven, titanium intramedullary lengthening nail, which is activated by applying an external magnetic field generator that causes a magnet inside the nail to rotate and effect lengthening.
Despite the technical advances made in the design and functioning of the intramedullary lengthening device and accurate control during lengthening and implant stability remain absolute prerequisites when performing limb lengthening, especially in the setting of bilateral lower limb lengthening. While the Albizzia® nail, G-Nail®, and Betzbone® permit full weight-bearing during lengthening, the Fitbone® and the P2 Precice® only allow partial weight-bearing (18 kg). A recent addition to the Precice® lengthening system, the Precice Stryde®, allows more weight-bearing depending on the nail diameter (10 mm allows 68 kg, 11.5 mm allows 90 kg, and 13 mm allows 113 kg).
This study aimed to investigate the mechanical properties of the various intramedullary lengthening devices currently available to orthopedic surgeons.
Materials and Methods
Nine intramedullary lengthening nails were used for mechanical testing: Albizzia® Ø 11 mm, ISKD® Ø 10.7 mm, Precice® Ø 10.7 mm, G-Nail® Ø 13 mm, and Betzbone® Ø 9 mm to Ø 13 mm. Of the tested nails, the Albizzia® and G-Nail® nails were manufactured from surgical steel, the ISKD® and Precice® nails from titanium, and the Betzbone® nails from cobalt-chrome.
A load cell connected to an electromechanical tensile testing machine Z600E (ZwickRoell GmbH, Ulm, Germany) was used to apply bending force on the nails. Each nail sample was placed on two 10 mm diameter supporting rods which were spaced 100 mm apart. Nails were placed so that the thickest outer diameter (female component) spanned the entire gap between the supporting rods [Figure 1]. A 12 mm diameter pushing rod applied a lateral bending force exactly the half distance between the supporting rods. The load was applied vertically at a rate of 2 mm/min until the desired deformation of each nail was reached. The force needed to deform each nail by 0.01 mm, 0.5 mm, 1 mm, and 3 mm was noted for each nail. During the first testing series, all nails were loaded to 0.1 mm deformation. Thereafter, all nails were loaded to 0.5 mm followed by 1 mm deformation. During the final testing series, all nine lengthening nails were loaded until 3 mm of deformation was reached. All tests were conducted using the same setup and in the same way.
Statistical analyses were performed using the SPSS for MAC version 27.0 (SPSS Inc., Chicago, Ill., USA). A simple descriptive analysis was done for each nail.
The minimum force required to deform a lengthening nail by 0.01 mm was 1266N (Betzbone® Ø 9 mm), whereas the maximum force required to deform a nail by 3 mm was 15639N (Betzbone® Ø 13 mm) [Table 1].
The resistance of nails increased with diameters. The Betzbone® Ø 9 mm needed the least amount of force to achieve 0.1 mm (1266N), 0.5 mm (3096N), and 1 mm (3523N) of deformation. The Betzbone® Ø 10 mm required an additional 1106N to be deformed by 0.1 mm (2372N), 2039N to be deformed by 0.5 mm (5135N), and 2245N to be deformed by 1 mm (5768N).
When viewing the results of the ISKD® Ø 10.7 mm, Precice® Ø 10.7 mm, Albizzia® Ø 11 mm, and Betzbone® Ø 11 mm together, the ISKD® Ø 10.7 mm consistently required the least amount of force to be deformed while, apart from 0.01 mm deformation where the Albizzia® Ø 11 mm performed the best, the Betzbone® Ø 11 mm was the best performing nail. To achieve 1 mm and 3 mm of deformation, the Betzbone® Ø 11 mm required between 60% and 80% more force than the ISKD® Ø 10.7 mm [Figure 2].
When viewing the results of the G-Nail® Ø 13 mm, Betzbone® Ø 12 mm, and Betzbone® Ø 13 mm nails, the Betzbone® Ø 13 mm consistently required more force to be deformed. Across all tests, the G-Nail® Ø 13 mm required the least amount of force to be deformed. When looking at only the 13 mm nails, the Betzbone® Ø 13 mm required between 50% and 80% more force than the G-Nail® Ø 13 mm [Figure 3].
Stainless steel and cobalt-chrome alloy nails were more resistant to deformation than titanium nails. When comparing surgical steel and cobalt-chrome alloy implants of the same size, more force was needed to deform the surgical steel nail for 0.01 mm, but for deformations beyond 0.01 mm, the cobalt-chrome alloy nails showed higher resistant to deformation [Figure 4].
The efficacy of any limb lengthening strategy relies on the ability of the lengthening device to accurately affect lengthening while providing adequate stability to support regenerate consolidation and functional rehabilitation. The aim of this study was to test the mechanical properties of contemporary intramedullary lengthening devices.
The principle finding of this mechanical study was first that the diameter is essential for the stability of lengthening nails. Similar to the findings of Penzkofer et al., the bigger the diameter of a nail is the higher the resistance against plastic deformation. Similarly, Lee et al. observed a higher percentage of nail bending in smaller diameter nails. Our findings also showed that the nail with the smallest diameter (Betzbone® Ø 9 mm) was least resistant to plastic deformation, whereas the lengthening nails with the larger diameter (Betzbone® Ø 13 mm and G-Nail® Ø 13 mm) showed the highest resistance against plastic deformation. From these findings, it is clear that surgeons should choose the largest diameter nail that would be accommodated in the diaphysis when performing lengthening procedures.
Comparing both Ø 13 lengthening nails, it seems that nonferrous metal is superior to surgical steel. Gotman investigated the characteristics of metals used in implants and described a higher wear resistance of cobalt-chrome-based alloys, which might explain our observations. Sahoo et al. and Goharian and Abdullah also described the properties of cobalt-chrome and emphasized the high specific strength of this alloy.
Interestingly, comparing the Ø 11 mm nails, the Albizzia showed a slightly higher resistance to plastic deformation of 0.01 mm in comparison to Betzbone®. In the test measuring the plastic deformation of 0.5 mm, 1 mm, and 3 mm, the cobalt-chrome alloy nail showed higher resistance to plastic deformation. Comparing the Ø 10.7 mm titanium nail to the Ø 10 mm and Ø 11 mm cobalt-chrome nails, the cobalt-chrome nails again performed better than the titanium nail, barring the 0.01 mm test where the Ø 10.7 mm titanium nail outperformed the Ø 10 mm cobalt-chrome nail. Goharian and Abdullah investigated the properties of titanium and cobalt-chrome alloys and found cobalt-chrome alloys to have higher resistance to axial compression, bending, and torsion. Our results support these findings as the cobalt-chrome alloy nail was found to show higher bending resistance, even when using a smaller diameter nail.
The clinical comparison of different intramedullary lengthening nails has only been evaluated in a single study. Thaller et al. compared the outcomes of different implants (ISKD® and Fitbone®) in a matched-pair analysis and found that the distraction index (mm/day) and the weight-bearing index (days/cm) was highly dependent on the specific technical drawbacks of each nail. However, no publication focusing on the mechanical properties of different lengthening nails has been published. This study emphasizes the importance of material properties and nail diameter when choosing one of these devices to perform intramedullary lengthening.
It should be kept in mind that the findings of this mechanical study do not necessarily translate to clinical relevance. We only performed lateral bending testing while more complex forces during gait are anticipated. A major limitation of the current study is the fact that only one nail of each size and manufacturer was tested, and this precluded statistical analysis. All available lengthening nails and nail sizes were also not included in the mechanical testing. All tested nails were also previously used to perform limb lengthening. We recommend repeating this study with a bigger sample size and included axial compression and micromotion in the testing protocols.
The intramedullary lengthening device's ability to resist bending deformation depends on the diameter and material of the nail. Surgical steel and cobalt-chrome alloy nails showed higher resistant to plastic deformation when compared to titanium nails. The identified differences should be borne in mind when choosing devices to perform intramedullary lengthening procedures. Whenever possible and according to the specific bone of the patient, higher diameters of implants should be used.
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Conflicts of interest
There are no conflicts of interest.
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