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READ OUR ARTICLE: LENGTHENING LONG BONES

Author: DR HAB. INŻ. DOMINIKA GRYGIER

One of the main manifestations of human motor activity is locomotion realized as gait. Its regularity and stable posture is derived from equal length of the lower limbs. Studies related to children’s posture conducted in the 1950s indicated that more than 90% of children under the age of 13 had body asymmetry in the frontal plane [1]. Ba- studies involving adults show, according to various sources, that limb length discrepancy (Fig. 1) affects this population between 40-70% [2-3]. For the most part, this is a slight difference that is compensated for by discrete deviations in anatomical structure in other regions of the body. The effects of significant limb inequality include not only general postural defects, such as curvature of the spine, lowering of the pelvis or the presence of a rib hump [4]. This defect is also associated with complaints of pain – with a particular focus on the L – S section of the spine [5].

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Figure 1 Limb-length inequality of more than 3 cm. Retrieved from https://orthoinfo.aaos.org/en/diseases-con- ditions/limb-length-discrepancy/.

TREATMENT METHODS

One method of preventing the onset of this dysfunction is to slow down or stop the process of growing longer bones. However, this method can only be used for children and adolescents at the adolescent stage. Another solution is surgical shortening of the longer bone. During such a procedure, a piece of bone is removed from the center of the longer limb, and the resulting fragments are then stabilized with bone plates. Nevertheless, this procedure can be performed only in the case of small discrepancies in limb length – in the case of the femur, such shortening reaches a maximum viable value of 50-60 mm [6]. The optimal method for resolving limb inequalities is surgical lengthening. The medical indication for limb lengthening surgery is limb inequality of more than 20-30 mm, dwarfism, and in the case of physically healthy patients, cosmetic and psychological indications [2, 7, 8].

According to Ilizarov’s research, it is assumed that the optimal value of daily lengthening is a distance of 1 mm, unless the patient’s condition does not allow it. Then the elongation rate is adjusted to the body’s capabilities (e.g., restoration of blood vessels in the elongation area), but does not exceed the stated optimal value [9]. Due to the nature of the medical tools used, these methods can be classified into two categories. The first method is the so-called Ilizarov method, which involves lengthening the limbs with an external apparatus [10]. It involves cutting the cortical layer of the bone and then placing an external apparatus to lengthen it (Figure 2). The cost of limb lengthening using an external apparatus is much lower than using internal lengthening methods. However, it should be noted that many potential patients opt out of the procedure using external braces after learning about the inconvenience and motor limitations resulting from their use. The above generates a trend away from external apparatus bone lengthening methods toward internal methods.

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Fig. 2 Lengthening of the fibula using the Ilizarov apparatus. Source: https://upload.wikimedia.org/wikipedia/commons/d/d9/Ilizarov3.jpg.

The internal method of bone lengthening is based on the use of an internal intramedullary device, which is surgically placed in the patient’s body after the bone has been dissected [11-13]. As the nail lengthens, the bones are gradually pulled apart and new bone is formed in the space created. This method does not use an external stabilizer, so it should be less burdensome for the patient: the entire healing process is painless. In addition, the implant placed inside the body does not impede the patient’s daily functioning. There is also much less risk of wound infection, which is often seen with external braces.

Bone lengthening with an internal brace avoids the physical and psychological challenges associated with an external stabilizer; however, both the internal and external limb lengthening processes take several months. Both procedures require regular follow-up visits to the doctor’s office and extensive rehabilitation, including physical therapy and home exercises.

Treatment options using intramedullary nailing carry certain requirements regarding the patient’s health, bone status, associated diseases and others. For this reason, there are several solutions on the market that differ in details of construction and operation. Manufacturers use three differing concepts: mechanical, electrical and magnetic, of which the mechanical solution has been the longest available on the market.

One of several mechanical bone lengthening concepts was developed by Guichet in 1990, originally under the name Albizzia® (Fig. 3). This solution (currently not commercially available) was a development of concepts already known since the 1950s. Activation of the implant and the associated lengthening was realized daily by the patient through 15 alternating outward and inward rotational movements of the limb by 20°. Due to mechanical activation with patients, often described as painful, the lengthening process encountered difficulties (the need for anesthesia during lengthening) and could ultimately prevent the planned length gain [13, 14].

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Figure 3 Schematic of the Albizzia® implant. Source: based on https://en.wikipedia.org/wiki/User:Dimmando/sandbox.

Very similar solutions were presented by the ISKD® (Orthofix®) design, launched in the US market in 2001. Here, too, the speed of the bone lengthening process was difficult to achieve, some patients complained of pain, the results of studies indicated poor quality of bone regeneration [14, 15]. And finally, due to numerous technical problems, spontaneous uncontrolled extension of the nail, lack of extension or blocking of the mechanism, the product was withdrawn from the market in 2011. Among electrically controlled implants, we should mention the German Fitbone® design (Fig. 4), which has been in use for about 20 years.

This is the fourth generation of nails manufactured by Wittenstein Intens®. This design allows both elongation and shortening of the bone. The elongation process is carried out using an external controller, whose impulses are received by a coil that is part of the implant. As a result, the internal drive system is activated, causing the implant to elongate or shorten. This design also has some drawbacks; problems with the implant hardware (wiring and antenna) and corrosion of the material used have been pointed out, and several patients have experienced osteolysis and swelling indicating inflammation [11, 15].

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Figure 4 Schematic of the Fitbone® implant. Source: based on https://en.wikipedia.org/wiki/User:Dimmando/sandbox.

Another group of products are elec- tromagnetically controlled nails. Implants of this type are most similar to the product under development by the Yu- ton Company. They are manufactured by two suppliers – NuVasive® (USA) producing the Precice® nail, which is available in Poland, and Phenix Medical® (France) producing the Phenix® implant. Both implants allow bi-directional length changes, providing linear feedthrough, which enables bone regeneration of better quality relative to implants that work oscillationally (Albizzia® and ISKD®). The Precice® system is a limb lengthening system with intramedullary nailing that has the ability to be remotely controlled by an electromagnetic system. However, due to the field’s range of action, overweight patients (BMI > 35) are excluded from the group of potential patients [16]. The system is used equally among patients with limb inequalities and those of short stature. The mobile intramedullary nail is equipped with an internal motor that is driven by a magnetic field. It is a minimally invasive and painless method. As a result of the treatment, it is possible to lengthen the bone by 5-6.5 cm.

Given the above, it should be concluded that there is a market demand for a dynamic method of bone lengthening that will allow this process to be carried out directly in the patient’s body, without the need for repeated surgical intervention. In addition, in order to identify patient needs, the Yuton Company conducted a series of interviews. Their results show that current patients primarily complain of intense pain complaints and significant discomfort during treatment.

The project carried out by Yuton Company includes R&D work that will result in the research and implementation of a hybrid magneto-mechanically driven intramedullary nail (Fig. 5) allowing the use of an active method of human long bone lengthening. The final product will be dedicated both to pa- tients with limb inequalities caused by congenital defects, developmental defects, deformities or resulting from accidents, and to able-bodied people whose short stature significantly reduces their comfort and quality of life.

The innovative dynamic intramedullary nail will be applicable to all patients – regardless of age and gender. The realization of the Project and the implementation of the results of R&D work on the market will contribute to the compensation of functional limitations resulting from different lengths of limbs, including mobility disabilities. The properties of the nail and the method of its active elongation in the patient’s body will allow to nullify the effect of bone demineralization, muscle atrophy and complications resulting from reduced joint mobility during the bone lengthening process. The indicated factors will allow faster therapy and easier recovery of patients.

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Figure 5 Concept of hybrid intramedullary nail with magneto-mechanical drive. Source: in-house development.

The main competitive advantage of the bone implant planned for implementation in the course of R&D work will be a hybrid magneto-mechanical mechanism enabling precise bone elongation. Such a solution will be possible through the use of a mechanism enabling bone elongation and the use of ferrite-free alloys and materials allowing the use of ma- gnetism phenomena for power transmission. The implant planned for development, thanks to the use of electromagnetic solutions, will enable bone elongation up to a length of 80 mm, with an accuracy of read elongation error < 15%.

The distinguishing feature of the Company’s Yuton product will be an integrated and simplified design that reduces the risk of damage or dysfunction. According to the assumptions, the mechanical and drive parts will be simplified relative to existing solutions, ensuring higher reliability of the target product. The structural strength of the implant, in contrast to current solutions, will allow for higher loading. The mechanical part will be developed to carry a physiological load of up to 80% of the patient’s body weight on both legs while maintaining a design life in fatigue cycles of min. 500,000 load cycles.

Research conducted as part of: R&D project “Intramedullary nail for active lengthening of long bones” Priority I: Support for R&D Works by Enterprises, Measure 1.3: R&D works financed with capital funds, Sub-measure 1.3.1: Support for R&D projects in the preseed phase by proof-of-concept funds – BRIdge Alpha. Support Agreement No. 1/2020/IGS

LITERATURE

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