Article focus

• To undertake macroscopical and microscopical evaluation of the Precice intramedullary nail system.

• To record differences following implant modification including levels of degradation and evidence of corrosion.

• To question a need for change in clinical practice and hypothesize the ability to reuse the implant in the same patient.

Key messages

• All implants demonstrated macroscopical and microscopical degradation.

• The degradation was less in the most recent implant design, but was not statistically lower.

• The earliest designs had evidence of internal corrosion and actuator pin damage, which was not seen in the latest design. There was, however, evidence of biological substances within the latest nails.

Strengths and limitations

• This is the first report of evaluation of the Precice nail implants and confirms an improvement in nail design following modification.

• It does not highlight a need for change in clinical practice and raises the potential issue of internal corrosion leading to actuator pin fracture if further lengthening with the same implant was to be considered.

• Only one nail of each design underwent formal micro-CT and subsequent sectioning to allow scanning electron microscopy and energy dispersive x-ray spectroscopy. Therefore, firm conclusions cannot be made regarding the effect to all nails.

Introduction

Limb-lengthening is a well-established orthopaedic practice following on from the concept of distraction osteogenesis pioneered by Ilizarov.^1-3 Traditionally, limb-lengthening is achieved using external fixators; however, complications, such as pin-site infection, joint stiffness, pain, deformity, and fracture following removal of the frame, have led to innovation in implant design. The use of an intramedullary nail has been successful in reducing external fixation application time and has led to intramedullary lengthening implants.^4-8

The latest device to be used is the Precice Intramedullary Limb Lengthening System (originally Ellipse Technologies Inc., Irvine, California; now NuVasive Inc., San Diego, California). This is a magnet-operated telescopic internal lengthening device with an outer casing of titanium alloy (Ti-6Al-4V). A cylindrical rare earth magnet is connected to a gear box and screw shaft assembly including an actuator pin within the nail. Two rotating rare earth magnets in an external remote controller (ERC) are held on the patient’s limb over the magnet within the nail. The resulting rotation of the implant magnet produces either desired lengthening or shortening with sub-millimetre accuracy.^9-14 The technology is based on the MAGEC rod (NuVasive Inc.) used as a ‘growth rod’ in the treatment of early-onset scoliosis.¹⁵

Since gaining United States Food and Drug Administration approval for lengthening of the femur and tibia in 2011, failures have been reported resulting in modification in the design of the nail.^16,17 The initial modular implant is now manufactured as a monobloc nail due to breakages occurring between the initial connecting segment welds and the internal mechanisms have been changed to produce a stronger, more robust device with an increase in the size of the actuator driving pin. The derotation ‘crown’ following breakage has also been removed in the latest design. MAGEC rod failures have been recently reported, however, with breakage and implant corrosion leading to tissue metallosis, adding caution when using these devices.^18-20

The aim of this study was to analyze Precice lengthening nails following retrieval, which was part of the routine treatment pathway. The retrieved nail was then analyzed both macroscopically and microscopically to evaluate any alteration to the nail and assess resultant differences following changes in implant design.

Materials and Methods

Retrieval analysis was undertaken on 15 Precice nails from 13 consented patients (approved by Health Research Authority, 07/Q0401/25). The details of the patients are shown in Table I. Femoral lengthening patients underwent extension of the nail at a rate of 0.33 mm three times per day; for tibial lengthening, the rate was 0.33 mm twice per day.

Indications for lengthening are listed in Table II.

The median femoral lengthening was 55 mm (interquartile range (IQR) 35 to 60) and the median tibial lengthening was 30 mm (IQR 27.5 to 40). All patients obtained the planned length with no implant failures. One tibial nail was removed prior to regenerate consolidation due to valgus deformity during lengthening. The deformity was corrected using fixator-assisted exchange nailing, with implantation of a Trigen Meta-nail (Smith & Nephew, Memphis, Tennessee). Consolidation occurred in all patients uneventfully with no requirement for bone grafting. No patients developed a superficial or deep infection. All the nails were removed following regenerate consolidation, which is recommended by the manufacturers, as the long-term effects of the magnet within the nail remaining in situ are unknown.

Implant analysis was undertaken using techniques previously reported in retrieved implant assessment.^19,21-23 Following implant retrieval, the first implanted nail was found to be fractured at the weld between the two parts of the nail, resulting in difficulty in retrieval of the distal part. All other nails were removed without complications.

Implant

Since inception, there have been three Precice nail implant designs. This study included six of the first-generation design (P1), while the remaining nine implants were second-generation (P2), further subdivided into four P2 and five P2.1 designs (Figs 1a to 1c).

Fig.

a) First-generation nail, shown by the presence of three welds and supported by the lot numbers referred to as P1. b) Second-generation nail, shown by the absence of welds and supported by the lot numbers referred to as P2. c) Second-generation nail, second modification, shown by the absence of welds and crown and supported by the lot numbers, referred as P2.1. d) Part of the crown on the actuator of a retrieved P2 (a magnification of the circled area of Figure 1b). e) Absence of the crown on a retrieved P2.1 nail (a magnification of the circled area of Figure 1c).

Results

All the implants had macroscopically visible marks on the telescopic part of the nail; these were directly related to the number of lengthening increments undertaken. Fretting and pitting were observed on these sites during microscopical inspection.

The evaluation of plain radiographs and micro-CT confirmed that all designs had common features including the magnet, gear box, and piston, including a collar with ball bearings. There were differences noted within the internal mechanisms, which are described in detail below.

Sectioning revealed actuator pin fractures in both P1 nails, while the P2 and P2.1 nails had intact pins. SEM revealed fretting and pitting on the internal parts, while EDS revealed corrosive debris in the P1 design. There were only biological deposits in the latest P2 and P2.1 modifications.

Macroscopical inspection

This revealed two different patterns of surface degradation on the telescopic part of the implants. All P1 implants sustained deeper marks, starting as narrow horizontal lines with wider horizontal lines close to the actuator part of the nail (Fig. 2a). Some of the implants had black debris on the markings. Biological tissue was located in the gap between the outer shell and the telescopic part of some of the implants.

Fig.

Patterns of damaged surface on the telescopic part of the nail. a) Retrieved P1 nail with wide marks on the telescopic part. b) Marks on retrieved P2.1, narrower compared with the marks of P1. c) Fretting (red arrow) and pitting (white arrow) corrosion present on the surface degradation on the telescopic part of P1 nail under the optical microscope (50 × magnification). d) Fretting (red arrow) and pitting (white arrow) corrosion present on the telescopic part of P2.1 nail shown by the microscopic inspection (50 × magnification).

In the P2 and P2.1 designs, the marks were less visible, and all appeared to be of equal width (Fig. 2b). The marks were noted to be deeper on the telescopic rod closer to the wider outer casing.

Plain radiographs

The lengthening recorded forensically and radiographically is presented in Table III. The state of the internal mechanism was further analyzed and the plain radiographs showed no signs of fracture of the internal mechanism. Representative radiographs from four implants are shown in Figure 3. Although all P1 nails had the same external appearance, radiographical analysis confirmed differences in the internal mechanisms. The implant identification numbers (lot numbers) that represented the earliest nails inserted were designed with a longer frame around the actuator pin, highlighted with a red circle in Figure 3a. Later implanted P1 nails with higher lot numbers were noted to have a smaller frame around the pin, the red circle seen in Figure 3b. The P2 and P2.1 internal design had two metallic rings surrounding the thread mechanism of the nail, highlighted by arrows in Figures 3c and 3d.

Table III.

Lengthening data of the implants from initial aim, achieved length, time of lengthening treatment, surface degradation from the forensic analysis, and the radiological estimation of the explanted nails

Design	Mean initial length aim, mm (range)	Mean achieved length, mm (range)	Mean time of lengthening, days (range)	Length achieved as measured by forensic methods, mm (range)
				Macroscopic inspection	Microscopic inspection	Radiological inspection
P1	37.0 (25 to 50)	35 (25 to 45)	44.50 (29 to 60)	24.50 (19 to 30)	27.50 (25 to 30)	31.00 (25 to 37)
P2	42.50 (25 to 60)	53.75 (25 to 65)	54.25 (24 to 69)	39.00 (20 to 58)	41.75 (20 to 60)	49.75 (26 to 60)
P2.1	33.75 (20 to 50)	38 (25 to 50)	52.80 (29 to 75)	27.00 (10 to 38)	33.40 (16 to 48)	36.70 (25 to 50)
p-value				0.4645	0.3342	0.5813

Fig.

Plain radiographs of different internal mechanisms in retrieved Precice nails. a) Radiograph of early P1 design, with a longer frame compared with the rest of the designs. b) Radiograph of the later P1 design with a smaller frame. c) Radiograph of P2 design with an arrow showing the metal rings close to the actuator pin. d) Radiograph of P2.1 design with an arrow showing the metal rings.

X-ray micro-CT

We selected four implants: two P1 nails (including one early and one late design), one P2 nail, and one P2.1 nail for further analysis. The scans confirmed the same differences between the designs, as the plain radiographs demonstrate in Figure 4; the black arrows highlight the metallic rings. The difference in size of the actuator pins can also be clearly seen.

Fig.

Micro-CT of different internal mechanisms/designs. a) Micro-CT of early P1 design with the arrow showing the increased frame. Comparing the pin with the micro-CT scans of the other designs shows a smaller pin, which sectioning revealed a fragment of the pin missing. b) Micro-CT of later P1 design with a larger actuator pin. c) Micro-CT of P2 and d) micro-CT of P2.1 designs revealed a similar internal mechanism. Presence of metal rings is shown with arrows.

Mechanical sectioning

The four nails were sectioned using plain radiographs and micro-CT scans as guides for the mechanical sectioning (see Table IV for the forensic analysis performed and the main findings). The internal mechanism of the early P1 design is featured in Figure 5, the P1 later design is featured in Figure 6, the P2 is featured in Figure 7, and the P2.1 is featured in Figure 8.

Table IV.

Forensic analysis of the four sectioned nails with the results from the imaging analysis, the detailed macroscopical inspection, the surface seen using scanning electron microscopy (SEM), and the elemental composition observed by performing energy dispersive x-ray spectroscopy (EDS)

Design	Radiographs / micro-CT	Surface damage on telescopic part	SEM	EDS
P1	Early design	Deep marks with different width, wider closer to the actuator part	Fractured site of the actuator pin, fretting and pitting corrosion, and presence of black debris on the parts of the internal mechanism	Presence of phosphorus and sulphur, high amount of oxygen, and titanium transfer
P1	Late design		Actuator pin fractured in two parts, fretting and pitting corrosion, and presence of black debris on the internal parts	Presence of sulphur, high amount of oxygen
P2		Shallow marks with same width	No pin fracture, fretting corrosion on the bearings, and presence of black deposits	Absence of phosphorus and sulphur
P2.1			No pin fracture, fretting corrosion on the bearings, pitting corrosion on the pin, and presence of black deposits	Absence of phosphorus and sulphur

Fig.

Microscopic inspection of internal parts of early P1 design. a) Microscopic picture of the ball bearing where black debris was found (arrow). b) Scanning electron microscopy (SEM) with presence of foreign materials on the ball bearing (arrow). c) Fractured site of the pin (arrow) showing a brittle fracture with no evident initiation or end points of the crack. d) SEM showed the rough surface of the fracture, combined with black debris on the side of the actuator pin (arrow). e) Black deposits on the plain bearings (red arrow) and fretting more prominent on the exterior part of the bearing (white arrow). f) SEM revealed black debris (red arrow) and biological material (fluorescent debris, indicated by the white arrow). g) The ball bearing previously situated at the bottom of the gearbox had a fat-like substance (arrow), most likely lubricant to facilitate motion within the internal mechanism. h) SEM confirmed the presence of fat-like substance (red arrow) and black debris (white arrow).

Fig.

Microscopic inspection of internal parts of a later P1 design. a) Non-fractured part of the actuator pin with presence of debris (arrow). b) Scanning electron microscopy (SEM) showed black debris on the side of the pin. c) Fractured site of the pin, with black debris, but with no clear indentation of the initiation and endpoints of the fracture. d) The initiation point was not evident using the SEM, suggesting a more brittle fracture. e) Black stained materials on the thread mechanism of the nail.

Fig.

Microscopic inspection of internal parts of P2. a) Foreign material on the pin (arrow) while b) scanning electron microscopy (SEM) showed no presence of black debris, only biological deposits on the side. c) Minor presence of black debris on the top of the pin (arrow), while d) SEM showed some black debris (red arrow) and pitting corrosion (white arrow). e) Ball bearing with debris and deposits (arrow), which f) under SEM inspection revealed presence of black debris (arrow).

Fig.

Microscopic inspection of internal parts of P2.1. a) Presence of foreign substances on the pin (arrow), while b) under the scanning electron microscopy (SEM) we observed black debris. c) The plain bearing had a small amount of foreign debris, which d) was more extended when we performed SEM (arrow). e) On the ball bearings, there was foreign materials (arrow), but not that extended, as f) SEM revealed. g) On the rubber ring, there was scratching on both sides (arrow).

All internal parts were found intact apart from the actuator pins of both P1 designs (Figs 5c and 6c). Black debris was found in all the sectioned implants on the threading of the telescopic part, as seen in Figure 6e. In the P2.1 example, a black rubber ring (Fig. 8g) was retrieved, which was not present in the other nail designs. It is presumed that this was inserted to prevent fluid ingress from the movement of the telescopic part during lengthening. There was scratching on the surface of the ring, denoted by the white arrow in Figure 8g.

The dimensions of the actuator pin range from 1 mm in diameter and 4 mm in length in P1 designs, to 2 mm in diameter and 6 mm in length in P2 designs.

Discussion

The Precice Intramedullary Limb Lengthening System is the latest implant in limb reconstruction surgery. This study has demonstrated that the early generation P1 designs had developed internal corrosive debris and confirmed actuator pin fracture, which was not recognized clinically. There was no corrosion debris seen internally in the later P2 and P2.1 designs, but the presence of biological deposits suggests that fluid ingress still occurred. Deposits of titanium wear particles were seen on all the internal stainless-steel bearings with degradation in the form of scratches seen on the smaller diameter telescopic part of the nail.

The Precice nail provides stability for the lengthened bone, allowing accurate controlled distraction and retraction whilst enabling continual physiotherapy to maintain joint movement. As an intramedullary device, the nail theoretically prevents regenerate deformity and fracture following weight-bearing. It has also been shown to have improved patient satisfaction and faster regenerate consolidation when compared with femoral lengthening using external fixators.¹² There has been modification of the initial design to strengthen the nail, following breakage through implant welds (P1) and subsequent crown failures (P2, initially designed to control rotation), which has led to the current P2.1. The actuator pins were also increased in size to withstand the forces required during lengthening.¹⁶ All these changes were demonstrated by the analysis performed in this study. With improvement in implant strength in the latest design, there has been an associated decrease in surface degradation with less visible markings, both macroscopically and microscopically.

There is also no evidence of corrosion debris within the P2 and P2.1 nails studied. The presence of biological deposits may suggest fluid ingress, which could contribute to corrosion occurring over time. This finding needs to be considered due to the combination of stainless steel bearings and actuator pin with a titanium casing, especially with all implants demonstrating titanium wear deposited throughout the internal mechanism.

Concern has recently been raised regarding the use of the MAGEC rods in the spine, due to the risk of implant failure.^18-20 High rates of failure and significant tissue metallosis surrounding the revised implants were associated with metal debris and fractures of the drive pin within the actuator.¹⁸ Panagiotopoulou et al¹⁹ reported skin breakdown and significant soft-tissue contamination as a result of the corrosive process. With the local effects of metallosis being highlighted further concern over the potential systemic effect has also been raised, with an increase in serum levels of titanium and vanadium found following treatment with the MAGEC system.²⁴

However, there are fundamental differences in the use of the MAGEC rod and the Precice nail. The ‘growing rods’ in scoliosis treatment remain in situ for several years, with intermittent lengthening undertaken as the child grows. In limb-lengthening, the Precice nail is used to obtain the length gradually but over a continuous period of up to 1 mm per day. Once the bone has fully consolidated, the nail’s removal is planned. It is therefore unlikely that a corrosive process will have sufficient time to cause an actuator pin fracture or other internal mechanism damage that compromises the use of the implant.

The Precice lengthening nail may perhaps also be considered for reuse in patients who require further lengthening within the same limb. The concept of the ‘dormant nail’ involves leaving the nail in place following initial lengthening and regenerate consolidation. The distal locking bolts can then be removed and the nail retracted using the ERC. The nail is then ready to undergo a secondary lengthening. The bone can be re-osteotomized and locking bolts replaced, and the nail undergoes a second lengthening episode. Another example is when the initial lengthening performed does not use the full availability of the nail. Dependent on the length of nail used, lengthening can be performed up to a maximum of 8 cm. In some patients this could be staged so that an initial lengthening is performed, for example up to 4 cm. Regenerate bone healing then occurs before a second osteotomy is performed to complete the subsequent 4 cm of lengthening. It is likely that in both these scenarios the nails will remain within the limb for several years rather than months. In these patients, there is a cost implication to be considered in using a new implant, as the Precice nail is expensive. Reusing the same nail, however, should be undertaken with caution, as our study suggests the possibility of internal corrosion leading to potential implant failure, although in the latest designs this may be low, as no corrosion was seen within the P2.1 implant studied.

There are limitations within this study. The implants were evaluated as they were retrieved from patients and so there are differences to the time in situ and differences in lengthening undertaken. Only one nail from each design has been evaluated with micro-CT, mechanical sectioning, SEM, and EDS. We are therefore unable to draw any firm conclusions that can be applied to all nails. We have demonstrated less degradation and no evidence of corrosive debris within the newer designs. We do, however, accept that there was no statistical difference between the degradation seen, and recognize the limitation of the small numbers analyzed, which could lead to a type 2 error. From the available evidence, the Precice nails all achieved the clinical outcomes of lengthening and subsequent good bone formation. Therefore, we would not advocate any change to current clinical practice. Further investigations are required to confirm the potential corrosion identified within the latest P2.1 design, and mechanical testing is needed to evaluate the potential of reusing the implant in secondary lengthening. Studies could also be performed to evaluate serum levels of titanium and vanadium to see if they are elevated during limb lengthening, as has been seen in the patients who had a MAGEC rod inserted.²⁴ This finding is especially important, as the P2.1 design has confirmed degradation with titanium deposits following lengthening.