Early Multicenter Experience With a Noninvasive Expandable Prosthesis

Abstract

A noninvasive expandable prosthesis for skeletally immature children after limb salvage surgery has been developed. Between 1998 and 2001, 18 Phenix prostheses were implanted in 15 pediatric patients who had been diagnosed and treated for osteosarcoma about the knee. Of the 15 original prostheses, 10 were implanted at the time of primary tumor resection and five were revisions from an endoprosthetic modular knee system. Sixty expansions of the 18 prostheses were done, with all but two done as outpatient procedures. An average 8.5 mm was obtained per lengthening (range, 1–30 mm). The average followup was 21.5 months (range, 12–33 months) and the latest Musculoskeletal Tumor Society functional scores averaged 90%. Eight revisions were required for stem fracture or loosening. There was one amputation because of a postoperative arterial thrombosis. The principle of the Phenix prosthesis involves storage of energy in a spring compressed by a locking system. Lengthening is achieved via exposure to an electromagnetic field that allows controlled release of the spring. This is a unique expandable custom prosthesis that offers many benefits in maintaining limb length equality in growing patients. Although the early experience is promising, additional data are required regarding the long-term structural integrity of the prosthesis. We are optimistic that this technology will prove beneficial, not only for patients with malignant bone tumors but in applications requiring serial limb length equalizations or for spinal deformities.

Limb salvage surgery for the skeletally immature patient with a malignant bone tumor poses unique surgical challenges for the orthopaedic oncologist. Osteosarcoma is the most common malignant tumor of bone seen in children. These tumors frequently involve the metaphysis of the distal femur or proximal tibia. ¹⁴ Local control of osteosarcoma often necessitates resection of the involved physis. More than ⅓ occur of all osteosarcomas occur in children younger than 10 years. ¹⁴ Continued unabated growth from the uninvolved, contralateral extremity can result in significant limb length inequality at skeletal maturity. A large number of patients will be faced with significant leg length inequality at maturity if limb-sparing surgery is done and future growth is not addressed.

In the past, surgeons have advocated amputation or rotationplasty as the treatment of choice for skeletally immature children with malignant tumors of bone that involved a major growth plate and an expected leg length discrepancy of 4 cm or greater. The eventual discrepancy is predicted based on the ablated physis location, age, and gender of the child. ^1,3,15 The potential for significant limb length difference and the inability to adequately compensate for the growth remaining are offered as reasons to avoid standard limb salvage techniques routinely done in older children with similar tumors. More recently, unique prosthetic devices have been designed to accommodate potential leg length discrepancy. ^{2,8,9,11–13} Modular designs afford the ability to return later and lengthen the prosthesis as the discrepancy develops. Exchange of a midsection component allows instant expansion of the prosthesis. The disadvantage of this procedure, however, is that it requires an open surgical procedure. Various authors ^4,6,8,11 have reported their results with this technique. The procedure frequently requires extensive surgical exposure and capsulotomy to exchange the midsection with a longer one. Complications such as neurovascular injury, infection, and postoperative stiffness are not uncommon. ^6,7,12–14

Noninvasive or minimally invasive designs have been described that avoid these complications. ^2,14,16 We report our experience with a noninvasive expandable prosthesis after tumor resection about the knee in skeletally immature children.

PHENIX EXPANSION MECHANISM

The Phenix is a custom-designed and custom-manufactured prosthesis comprised of multiple components (Phenix-Medical, Paris, France; now manufactured as Repiphysis, Wright Medical Technology, Arlington, TN). The expansion capability is based on the length of tumor resection, anticipated future growth of the contralateral extremity, and potential limb length discrepancy at skeletal maturity.

The expandable body of the Phenix is based on a Ti tubular part that fits inside a polymeric tube (Fig 1). At one extremity of the Ti tube, an annular protuberance penetrates the polymeric tube, locking these two interlocked parts together. The exterior body is made of poly ether ether ketone with a PE tubular insert maintained by a bolt. The closed extremity is connected to the Ti stem, which is intended to link the prosthesis to the residual bone. A spring compressed between the two closed cylinders stores energy relative to its length, diameter, and cubic thickness.

Fig 1: A lateral rendering shows a custom Phenix femoral prosthesis. The expandable body is based on a Ti tube (A) that fits inside a polymeric tube (B). The exterior body (B) is made of poly ether ether ketone with a PE tubular insert maintained by a bolt. The closed extremity (A) connected to the Ti stem links the prosthesis to the residual bone. A spring (S) compressed between the two closed cylinders (A and B) stores energy relative to its length, diameter, and cubic thickness. During a lengthening procedure, the annular protruberance (L) is heated selectively by an induction method using an electromagnetic field. (Reprinted with permission from Wilkins RM, Soubeiran A: The Phenix expandable prosthesis: Early American experience. Clin Orthop 382:51–58, 2001.)

When an expansion is required, the annular protuberance is heated selectively by an induction method in which a magnetic field created by a coil and its specific generating current is used. The resultant magnetic field must be applied at the proper location 15 to 30 seconds to achieve a significant result. Such conditions would not be encountered in the patient’s normal environment.

Once heated to more than 130°C, the PE that is in contact with the protuberance melts, allowing the control of the progressive slide out of the Ti tube from the polymeric one under the spring force. Only the volume of PE that the protuberance has to go through is melted during the expansion procedure. A given section of the PE tube is melted only once during the lifetime of the prosthesis. The heat generated for the melting process then is dissipated slowly through the entire implant, the surface of which is cooled by the blood flow so that the surrounding local tissues are not heated significantly. For example, the increase in temperature in the Ti column is less than 3°C during a typical lengthening when measured in air. When the magnetic field is stopped, the entire system cools and the polymeric tube and interior Ti part are linked rigidly again, albeit in a new position. Between 6 and 20 mm of length can be obtained during each expansion depending on the patient, the compliance of the soft tissues during the procedure, and the spring residual force. Lengthening of more than 20 mm should be avoided to prevent potential neurovascular injury.

MATERIALS AND METHODS

Between 1998 and 2001, 18 noninvasive expandable prostheses were implanted in 15 patients (Table 1). All procedures were done by the authors at one of two centers. All patients had a diagnosis of high-grade, nonmetastatic osteosarcoma. To be considered a candidate for placement of the noninvasive prosthesis, the patient had to have an existing or anticipated limb length discrepancy greater than 2 cm and be considered a candidate for limb salvage. Informed consent and formal protocol criteria as established by local institutional review boards were met in all cases. Approval for compassionate use was granted on a case-by-case review basis by the United States Food Drug Administration.

TABLE 1: Patient Data

Ten prostheses were implanted at the time of primary tumor resection and five prostheses were conversions from preexisting modular systems. These five patients received a modular oncology endoprosthesis at the time of the original tumor resection and had been receiving periodic lengthenings requiring a midsection exchange during a major surgical procedure. As the new noninvasive technology became available, the decision was made to convert the endoprostheses to the Phenix device for the duration of the patients’ active growth. Three patients had subsequent revision of their original Phenix device because of failure of the expandable portion of the device and had revision surgery to a second Phenix device. All patients met the inclusion criteria for the current study. Therefore, 18 devices were implanted in the 15 patients.

Each Phenix prosthesis was custom-designed based on preoperative imaging. Consideration was given to length of resection, growth potential, and future leg length discrepancy. An attempt was made to individually tailor the calculated maximum expansion into each device. For the five patients who had revision surgery from an endoprosthesis to a Phenix prostheses, the original manufacturer was consulted to ensure conformity between the existing well-fixed stem and the Phenix device. The articulating segment was removed and the stems were left in place. The Phenix expandable body and hinge then were secured to the preexisting components.

The sites of implantation included one total femur, 10 distal femurs, and four proximal tibias. There were nine boys and six girls. The average age of patients at the time of Phenix implantation was 11 years (range, 7–15 years). The average age for the boys was 11 years (range, 7–14 years) and the average age for the girls was 11.5 years (range, 9–15 years).

The same basic surgical technique was used in all patients having primary surgery. The stem on the resected side of the joint was cemented in place using a standard technique. On the uninvolved side of the joint, the stem was press-fit. Derotation fins on the base plate of the device provided additional fixation.

Postoperatively, patients with distal femoral resections had standard rehabilitation with early aggressive active and passive physical therapy, CPM, and early full weightbearing when tolerated. The four patients (Patients 3, 8, 15, 17) who had proximal tibial replacements also had gastrocnemius muscle flap coverage and extensor mechanism reconstruction. The limbs of patients with proximal tibial resections were placed in full extension for 6 weeks postoperatively to protect the extensor mechanism repair and the patients were restricted from doing any knee flexion. Active and passive ROM exercises then were instituted. In patients receiving adjuvant treatment, chemotherapy was completed per protocol without incident or delay.

Lengthening Procedures

During routine postoperative followup, standard radiographic images of the prosthesis were obtained. These included AP and lateral radiographs and, when appropriate, scanograms of both extremities. The plain radiographs ensured the integrity of the device and fixation. The scanograms were done to exactly calculate the leg lengths and to determine any discrepancy. Once the patient had a significant discrepancy (>0.5 cm) and the patient had acceptable ROM (approximately 0°-90°), a lengthening procedure was scheduled.

Lengthening procedures were done as outpatient procedures in the radiology department. A mild oral analgesic generally was administered if the patient was overly anxious or uncomfortable. Only two patients required general anesthesia because of emotional problems and an inability to cooperate while awake.

Patients were placed on a radiolucent, fluoroscopy table and the entire prosthesis was imaged to locate the annular protruberance and the mobile portion of the implant. When the annular protruberance was located under fluoroscopy guidance, a mark was made with a skin marker to identify its location within the body of the implant (Fig 2). A second image was obtained to observe the mobile portion of the expansion component. There is a barrel on the expansion component from which the modular portion emerges. The distance between this barrel and stem was measured (Fig 3A). As the mobile portion of the prosthesis expands out of the expansion component, this interval increases. Therefore, a measurement was taken with the fluoroscopic ruler and this length was checked after each activation to determine the exact expansion length (Fig 3B-C).

Fig 2: A–B. (A) An AP view shows the knee of an 11-year-old boy with an osteosarcoma of the proximal tibia. The postoperative view shows the hinge and expansion mechanism of the Phenix prosthesis. The proximal arrow points to annular protruberance. The distal arrow indicates the mobile portion of the prosthesis at the barrel-stem junction. (B) A close up view shows the annular protruberance (arrow). The activating coil is placed over this area.

Figure: Continued.

Fig 3A: –C. (A) An AP radiograph shows the expandable portion of the Phenix prosthesis. The Ti tubular portion expands out of the polymeric tube at this portion of the implant (arrow). On this initial postoperative radiograph, there is not a perceivable interval between the barrel and the bone. (B) The distance between the barrel and the stem of the implant is measured before activation. The interval radiograph shows prior expansion. There is an increase in distance between barrel and bone (arrow). (C) The increased distance between the barrel and stem indicates the amount of expansion that occurred.

Figure: Continued.

Figure: Continued.

To activate the device and create expansion, the electromagnetic coil was placed about the extremity and held in place by the surgeon. The coil was calibrated and tested before each procedure. It was positioned directly over the annular protruberance as indicated by the skin mark. The coil was activated via a push button thereby activating a magnetic field. The coil was held in place by the surgeon who also monitored the activation box. A second assistant monitored the time of activation. The patient, a family member or the surgeon activated the push button. Most patients activated the device themselves (Fig 4). Each activation lasted between 20 and 30 seconds. Initially there was no discomfort. Then, as expansion occurred and the soft tissues were pulled, the patients sometimes described a stretching or pulling sensation and some discomfort. Occasionally, there was warmth in the leg. The patients describe the discomfort as about the same as a physical therapy session.

Fig 4: This photograph shows a 13-year-old boy during activation of Phenix expansion. The activating device is located to the left. The electromagnetic coil is being held over the annular protuberance within the implant.

Fluoroscopy was used to determine how much expansion occurred after each activation by measuring the interval between the barrel and the stem. In general, the goal of each lengthening was to achieve between 5 and 20 mm expansion. In almost all cases, multiple repeated expansions (average, 2–3) were required to achieve the desired goal. In some cases, such as with a patient having an active growth spurt or a patient who must travel a distance to the hospital, an attempt was made to overlengthen by 5 mm rather than merely equilibrate the discrepancy. After activation, the patient’s neurovascular status was evaluated. After completion of the procedure, the patient was checked again for neurovascular integrity and final radiographic images were obtained to document expansion length. The patient was observed for a short period (15 minutes) in the outpatient clinic and then discharged. Continued aggressive ROM was used to maintain full ROM. The followup schedule was planned according to a routine limb salvage protocol. More frequent images were obtained if the patient complained of a new growth discrepancy during the interval.

RESULTS

Eighteen Phenix prostheses were implanted in 15 patients, of which 10 were the original device implanted in the patient. Three were second Phenix devices implanted after failure of the expandable portion of an existing Phenix prosthesis.

Of the original 15 prostheses, 10 still are in place in patients who are having active expansion. Eight revision procedures were required and one above-knee amputation was necessitated for an arterial thrombosis at 10 months postoperatively. The average time from implantation of the 18 Phenix prostheses to either failure or last followup was 18 months (range, 6–33 months). The average followup for patients with a functioning prosthesis after expansion was 22 months (range, 14–33 months).

Sixteen of the 18 prostheses in 15 patients have undergone 60 lengthening procedures. One patient required an amputation before lengthening and a second patient had revision of the Phenix prosthesis 6 months ago and has not yet had an expansion. To date, patients have averaged 4.3 lengthening procedures, with an average 8.5 mm length obtained during each procedure (range, 1.5–30 mm). Mechanical difficulties resulted in only one failure to lengthen. There was no neurovascular compromise or significant loss of motion. All patients regained their preexpansion functional level.

Functional evaluations were done before and after expansion and during routine followup. The MSTS scoring system ¹⁰ averaged 90% for all patients at most recent followup. For the 10 patients with a functioning prosthesis in place who are undergoing active expansion, the average MSTS score was 93.5%. Five of 10 patients completed Hospital for Special Surgery knee questionnaires at their last visit and the average score of 88% closely approximated the MSTS scores.

Ten patients still are undergoing active expansion. Of the three patients who have reached skeletal maturity, all have a 1 cm or less leg length discrepancy. One patient with a 6-cm discrepancy has progressive disease and refused lengthening until after chemotherapy is completed.

Revisions

Seven patients required eight revisions; three of these patients had revision to a second Phenix prosthesis. The first patient to undergo Phenix implantation (Patient 1) had conversion from a modular endoprosthetic knee tumor system. His tibial component required two component revisions because of fracture; the patient then had revision to a second Phenix prosthesis and, once skeletal maturity was achieved, he had revision to a conventional endoprosthetic total knee arthroplasty. A second patient (Patient 4) was an active high school cheerleader who was expected to reach a height of 6 feet or more. She experienced loosening of the rotational body of the Phenix prosthesis after having total lengthenings of 56 mm and had revision to a second Phenix prosthesis. The third patient (Patient 11) had revision to a second Phenix prosthesis after fracture of the expandable component sustained during a fall from a scooter. He subsequently had a metachronous lesion develop in the contralateral distal femur and refused any additional lengthening procedures until completion of chemotherapy. During this period, a stiff knee and significant shortness developed.

Two component revisions (Patients 8 and 14) were required because of hardware failure at 16 and 9 months postoperatively. Another patient (Patient 6) experienced loosening at 28 months and had revision to an allograft-prosthetic composite because she had reached skeletal maturity. An amputation was required in because another patient (Patient 9) 10 months postoperatively. This patient had several debridements for wound healing problems and revision of a free flap, but then had an arterial thrombosis and ischemic necrosis of her lower extremity, which necessitated a knee amputation.

Six revisions in five patients were required secondary to prosthetic failure, four of which were for failure of a nonexpandable portion of the device. The first patient (Patient 1) had revision to a second Phenix prosthesis. The second and third patients (Patients 8 and 14) retained the expandable portion and had revision of the fractured stem or hinge plate portion only.

Two patients (Patients 3 and 11) had failure of the expandable body component comprising the spring and locking mechanism after significant trauma (Fig 5). Neither patient who sustained fracture of the expandable body after falling had any neurovascular compromise. Both patients had approximately 2 cm of acute lengthening at the time of the injury. At the time of revision, one patient still had some compression of the spring. The first patient had revision to a conventional knee replacement and subsequently died of disease. The second patient had revision to a new Phenix expandable component. He had achieved greater than 6 cm of expansion before his fall and was functioning well. After placement of the second Phenix prosthesis, he had two expansions achieving 1.5 cm. He has refused additional expansions or interventions until he completes chemotherapy.

Fig 5: The photograph shows a retrieved specimen with a fracture through the nonexpandable portion of the device.

DISCUSSION

The treatment of malignant bone tumors in the skeletally immature patient has long been a challenge to orthopaedic oncologists. Resection of osteosarcoma frequently requires sacrificing the adjacent physis of the involved bone. Very young patients with significant growth remaining can expect a major limb length inequality at maturity. Reconstruction with an endoprosthesis often is used to regain joint mobility. Unabated growth of the contralateral extremity results in a significant limb length inequality at maturity. Reconstruction with a prosthetic design that causes ablation of the uninvolved side of the joint’s physis only adds to this discrepancy. For example, a 10-year-old boy with a distal femoral lesion has resection of the distal femoral physis. Reconstruction frequently ablates the proximal tibia growth plate. The loss of both growth plates results in a loss of 1.5 cm per year (1 cm from the distal femur and 0.5 cm from the proximal tibia). If the patient reaches skeletal maturity at age 16, he will have almost a 9-cm discrepancy. Endoprosthetic designs have addressed this issue with one of three major types of designs, including (1) modular designs in which midsections are exchanged for progressively longer sections; (2) a minimally invasive design that requires a small incision to expand the prosthesis and; more recently, (3) noninvasive designs for expansion. ⁵ Early experience with the specific technology described by us has been reported elsewhere. ¹⁶

Significant complications occurred in seven patients. Four of these were for failure of the device and three were after significant trauma. One patient was markedly overweight, which was thought to contribute to the hardware failure of the hinge mechanism. Three patients had reached skeletal maturity or maximal expansion and had revision to a conventional TKR. Of the seven patients who required surgery after failure, five had successful revision to a functional prosthesis and two had revision to a functional TKR. Three patients currently are undergoing active expansion with the Phenix device. One patient died from disease and one patient is struggling with problems related to disease progression.

This prosthesis initially was used to provide skeletally immature patients with a noninvasive method of prosthetic expansion. Maintaining equal leg lengths at maturity or equilibrating the existing extremity without invasive surgery was the rationale for the use of this device. All patients had successful noninvasive lengthening. Patients undergoing active expansion, or those who have completed expansion, have less than 1 cm discrepancy in leg lengths. In most cases, preexisting discrepancies were corrected. The majority of lengthenings were done in the fluoroscopy suite with patients requiring only oral analgesics for minor pain. Patients who experienced a midsection exchange-type of expansion in the past with a preexisting prosthesis were pleased with the new technique. The functional result is comparable with that seen in the modular endoprosthetic population in this age group and in comparison with older age groups.

As with any new design, corrections and improvements have been made with the experience gained through adversity. The complications inherent with loosening of the rotational body and with pairing an existing fixed stem to an expandable body have been addressed. Since the last report on the Phenix prosthesis, ¹⁶ several modifications have been implemented because of problems with the expansion mechanism. Eventually this expansion mechanism will be integrated into a modular oncology system (Guardian, Wright Medical Technology, Arlington TN).

The goal of noninvasive prosthetic expansion for skeletally immature patients has been achieved with the current design. We update the early results of the still-evolving prosthetic design used by one surgeon (RMW) with an improved design used for 3 years by the coauthors. The experience described focuses on reconstruction of the knee with the Phenix prosthesis; however, we also have had similarly good results in applying this technology to the proximal humerus, proximal femur, and ulna. We think that this expandable prosthetic design shows promise in handling the difficult problem of limb preservation in a growing child.