ALTHOUGH RARE IN THE general population, osteosarcoma

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1 1039 ORIGINAL ARTICLE Functional Outcomes After Limb-Salvage Surgery and Endoprosthetic Reconstruction With an Expandable Prosthesis: A Report of 4 Cases Kathleen Beebe, MD, Kimberly J. Song, MA, Ellen Ross, PhD, PT, Benjamin Tuy, MD Francis Patterson, MD, Joseph Benevenia, MD ABSTRACT. Beebe K, Song KJ, Ross E, Tuy B, Patterson F, Benevenia J. Functional outcomes after limb-salvage surgery and endoprosthetic reconstruction with an expandable prosthesis: a report of 4 cases. Arch Phys Med Rehabil 2009; 90: Objective: To determine the functional outcomes of skeletally immature patients after replacement of the femur and tibia performed by using noninvasive expandable endoprostheses. Design: Case series. Setting: A hospital-based ambulatory care center. Participants: Pediatric patients (N 4) with primary bone tumors of the distal femur and proximal tibia who underwent surgical replacement performed by using the Repiphysis noninvasive expandable endoprosthesis (Wright Medical Technology, Memphis, TN). Interventions: Wide resection of bone sarcoma and placement of expandable endoprosthesis. Main Outcome Measures: Musculoskeletal Tumor Society (MSTS) scores were assessed at the beginning of the study and at each follow-up visit. Medical Outcomes Study 36-Item Short-Form Health Survey, Version 2 (SF-36); gait; sit-tostand transition; and range of motion (ROM) were assessed at an average follow-up of 31.5 months. Results: At an average of 31.5 months postoperative, the SF-36 physical component summary scores lagged behind the national mean, whereas the mental component summary scores were satisfactory. MSTS scores indicated low levels of pain and supports use with high emotional acceptance and walking ability but persisting difficulties with function and gait. Patients also showed altered patterns of sit-to-stand transition including decreased peak vertical force in the operated limb and increased center of mass momentum in a shorter amount of time. Parts of gait functioning were found to be decreased, including gait velocity, stride length, and cadence. Some patients displayed alternate weight-bearing strategies that accompanied increased double-limb support and stance phase during walking. ROM and strength were diminished at both the hip and knee joints in the operated limb and in the nonoperated limb. Conclusions: Reconstruction with a noninvasive expandable endoprosthesis produces satisfactory functional outcomes in pediatric patients with primary tumors of the bone. Patients in our study displayed some persisting physical difficulties including decreased ROM and strength and altered gait and sit-to-stand patterns, yet they maintained high levels of emotional acceptance and coping. Key Words: Limb salvage; Lower extremity; Outcome assessment (health care); Rehabilitation by the American Congress of Rehabilitation Medicine ALTHOUGH RARE IN THE general population, osteosarcoma and Ewing sarcoma are the most common primary malignant bone tumors in children. 1 Osteosarcomas originate most frequently in areas of rapid bone growth, such as the metaphyseal regions of the distal femur, proximal tibia, and proximal humerus of adolescents. 2,3 Because of a high tendency to metastasize, treatment for osteosarcoma must be aggressive and usually requires both neoadjuvant and postoperative chemotherapy in addition to surgical resection. Approximately 225 new cases of Ewing sarcoma are diagnosed annually in children and adolescents in North America. 4 Although these malignancies can arise in virtually any bone or from soft tissue, the most common primary tumor sites for Ewing sarcoma are the pelvis, femur, tibia, and ribs. With Ewing sarcoma of the long bones, malignancies tend to arise from the diaphyseal rather than metaphyseal locations. 4 Before the 1970s, amputations were the preferred treatment for malignancies of the extremities, yet still most patients died of metastatic disease. 3 With the introduction of adjuvant chemotherapy, major improvements in survival have been seen, 5 and advances in surgical technique have permitted the sparing of limbs and greater overall survival for most patients. Because skeletal sarcomas rarely directly invade the nerves or vasculature, the involved bone and adjacent affected muscle can be excised, leaving a neurovascularly intact limb that can be reconstructed in one of several ways. Early endoprostheses were custom-made alloy of high strength and low weight, often replacing the entire involved bone and incorporating a simple From the Departments of Orthopaedics (Beebe, Song, Tuy, Patterson, Benevenia) and Physical Therapy (Ross), University of Medicine and Dentistry of New Jersey, Newark, NJ. Deceased. Presented to the American Academy of Physical Medicine and Rehabilitation, November 20 23, 2008, San Diego, CA. Supported by the Foundation of University of Medicine and Dentistry of New Jersey (grant no ). Reprint requests to Kathleen Beebe, MD, UMDNJ Ambulatory Care Center, 140 Bergen St, Ste D1610, Newark, NJ 07305, beebeka@umdnj.edu /09/ $36.00/0 doi: /j.apmr CDF COM GC MCS MSTS PCS QOL ROM SF-36 List of Abbreviations continuously disease free center of mass gait cycle mental component summary Musculoskeletal Tumor Society physical component summary quality of life range of motion Medical Outcomes Study 36-Item Short-Form Health Survey, Version 2

2 1040 OUTCOMES USING AN EXPANDABLE PROSTHESIS, Beebe hinge joint. 6 Newer endoprostheses are modular and off-theshelf, allowing reconstruction to be performed without delay, an advantage over other custom-designed implants. Although cadaver allografts and allograft-prosthetic composites are important biological reconstruction with potentially favorable long-term results, they are nevertheless associated with a high rate of early postsurgical complications including infection, fracture, and nonunion. 7 Rotationplasty, which can be performed when the sciatic nerve is preserved, 8 effectively converts the ankle joint into the knee joint to create a functional below-knee amputation 9 and is a low-cost alternative to custom-made endoprostheses. 10 However, the resulting cosmetic appearance is often dissatisfying to the patient, 11 and there are associated risks because of increased biomechanical stress at the ankle joint. 12 With the decline of amputation and the progress of successful, durable reconstructions through the previously mentioned options, the focus of treatment can now be shifted toward other issues in cancer rehabilitation. For the many pediatric and young adult patients with malignancies of the extremities, it is important to address the challenge of limb-length discrepancies. This is a significant concern especially for skeletally immature children and adolescents with much growth remaining because natural postsurgical growth of the contralateral limb results in a length difference between the extremities. Expandable modular endoprostheses were developed as a means of overcoming such limb-length discrepancies. In the 1980s, the Lewis Expandable Adjustable Prosthesis a was introduced, allowing surgeons to surgically revisit a previously implanted endoprosthesis and administer periodic invasive expansions. This permitted gradual increases in the salvaged tissue of the affected extremity and allowed it to eventually be of comparable length to the contralateral limb. Studies 13,14 performed by using the Lewis Expandable Adjustable Prosthesis on children with Ewing sarcoma or osteosarcoma have reported positive results. The necessity for repeat invasive surgical procedures, however, carries the risk of infection and potential morbidities from perioperative emotional and physiologic stress. More recently, an endoprosthesis originally known as the Phenix was developed by Phenix Medical in France and is now produced under the name Repiphysis. a This device allows for periodic noninvasive expansions via the release of a spring using carefully applied external electromagnetic fields. 15 Essentially, the Repiphysis is composed of a titanium tube inserted within a polymeric tube, with an annular protuberance at 1 end locking the 2 parts together. A spring is compressed between the 2 tubes, storing energy 15 (fig 1). Lengthening occurs when an external magnetic field is applied over the annular protuberance, heating a polyethylene insert and allowing the spring to slowly expand the length of the prosthesis. Local blood flow helps to dissipate the generated heat, preventing damage to the surrounding tissues. Although reported expansion rates have varied, 15 to 30 seconds of electromagnetic field activation should achieve expansion targets of about 6 to 15mm. 15 Previous studies involving the Repiphysis have shown positive results, although functional analyses have largely been limited to scores on the MSTS evaluation system. A study by Neel et al 16 followed 18 endoprostheses implanted in 15 pediatric patients over the course of 60 total expansions and found that the patient population scored an average of 90% on the MSTS scale at an average follow-up time of 21.5 months. In another investigation by Wilkins and Soubeiran, 15 6 patients underwent a total of 21 lengthenings and experienced satisfactory mechanical functioning, with an average MSTS score of 80% at a mean follow-up of 14 months. The MSTS system, which was developed in the 1980s by the MSTS and formally adopted by the International Symposium on Limb Salvage, aims to efficiently evaluate the functional outcomes of patients who undergo limb-salvage or ablative procedures for malignancies of the extremities. MSTS scores are assigned on a scale of 0 to 5 in each of 6 categories, with a possible total score range of 0 to 30. For the lower extremity, these categories are comprised of pain, function, emotional acceptance, supports, walking ability, and gait. 17 Although the reliability, validity, and responsiveness of the MSTS are unknown, it has been widely accepted and used in evaluating limb-salvage patients. 18 However, it has been noted that its scores may not correlate with actual measures of function or health-related QOL, especially because patient functioning is rated by the observer or clinician, thus introducing external subjectivity. Studies have shown that the MSTS system fails to identify significant differences in function that are recognized only by requiring patients to physically perform specific tasks 19 and have found only moderate or no correlations between MSTS scores and those obtained by the SF-36 or objective measures of function such as timed up-and-down stairs, Timed Up & Go, and timed run-walk tests. 20,21 The SF-36 is a self-administered questionnaire that provides insight into health-related QOL and allows the patient s own views on his/her physical and mental functioning to be incorporated into the rehabilitation plan. On the SF-36, patients rate themselves on a PCS scale and an MCS scale, with higher scores indicative of better health. The PCS consists of physical functioning, role limitations due to physical problems, bodily pain, and general health perception subscales, whereas the MCS is comprised of vitality, social functioning, role limitations due to emotional problems, and mental health subscales. Fig 1. (A) Schematic of an implanted Repiphysis prosthesis with a magnetic field being applied around the patient s leg. (B) As the magnetic field is applied, the spring is slowly allowed to expand the length of the Repiphysis. (Reprinted with permission from Wright Medical Technology, Memphis, TN.)

3 OUTCOMES USING AN EXPANDABLE PROSTHESIS, Beebe 1041 This system has been validated and found to be a reliable and reproducible means of evaluating functional outcomes Using both the MSTS and SF-36 along with other methods of functional evaluation may allow for more information on the outcome of patients treated for musculoskeletal malignancies. In this study, we advocate the use of the MSTS and SF-36 to assess the functional outcomes of 4 patients who have undergone limb-salvage surgery with the Repiphysis as well as subsequent lengthening and rehabilitation. We also incorporate other objective measurements, including strength, ROM, gait, and sit-to-stand movements. Comparing bilateral strength and ROM at the hip and ankle joints allows us to examine the relative abilities of the affected lower extremity with regard to both the contralateral limb as well as normalized values found in healthy people without endoprosthetic replacement. Furthermore, because walking and transitioning from a seated to standing position comprise a fundamental part of daily living, an objective functional analysis of these movements provides valuable information on the most essential targets of physical therapy for our patients. By means of these measures, we obtained in-depth subjective and objective information regarding patients physical and emotional outcome after surgery performed by using the Repiphysis noninvasive expandable endoprosthesis. METHODS Four skeletally immature patients with sarcomas around the knee underwent tumor resection and reconstruction with the Repiphysis. Each patient underwent lengthening of the prosthesis and was asked to participate in this study. Patients were excluded if they were not at least 1 year out from surgery (to ensure that functioning had been maximized), and patients who received lengthening treatments but did not survive to the time of study enrollment were also excluded (only 1 additional patient at our institution was part of this exclusion group). Three of the included patients had osteosarcoma, and 1 had Ewing sarcoma. The primary sites of disease included the distal femur in 3 patients and the proximal tibia in 1 patient. When feasible, expansion procedures were scheduled for each patient when he/she developed 1-cm limb-length discrepancy based on follow-up x-ray scanograms. All expansions were performed under anesthesia in the operating room, with fluoroscopic guidance. MSTS, SF-36, and objective functional evaluations were performed at an average follow up of 31.5 months (range, 15 42mo). Objective functional evaluations included parameters of gait, sitto-stand motions, ROM, and strength. Patients meeting the study criteria were asked to voluntarily participate in the study after completing rehabilitation, and institutional review board approval was obtained. All risks and benefits of the study were explained to each patient, and consent was obtained. Muscle strength was measured by using a hand-held dynamometer, and the average of 2 trials was used. ROM was measured manually by using a goniometer. Strength and ROM were measured for the following motions bilaterally: hip flexion, hip extension, hip abduction, knee flexion, and knee extension. Goniometry and dynamometry have proven to be reliable and valid methods of measuring ROM and strength, respectively. The kinematics and kinetics of the sit-to-stand motion were analyzed by using the Peak Motus Motion Analysis System b and 2 Kistler force plates. c Subjects were seated with reflective markers placed over the acromion, greater trochanter, lateral joint line of the knee, and lateral malleolus bilaterally and then asked to perform several repetitions of the sit-to-stand motion (fig 2) while 6 cameras recorded the movements and the Peak Motus software synchronized the data. Gait analysis was performed by using the GaitRite system, d consisting of a portable mat containing a matrix of pressure sensors connected to a laptop computer. Patients were instructed to walk several times on the mat while temporal-spatial parameters were automatically recorded (fig 3). SF-36 data were compared with national means, ROM was compared with normalized values for all ages, 31 and strength was compared with normalized values for all ages as specified by the American Academy of Orthopaedic Surgeons. Sit-tostand data were compared with normalized values obtained from young adults years of age, 32 whereas gait data were compared with normalized values obtained from healthy adults and children Case Descriptions Patient 1. Patient 1 was 9 years old at the time of surgery and 12 years old at the time of functional evaluation. She was diagnosed with osteosarcoma of the right distal femur and underwent distal femur replacement in May Until the time of evaluation at 36 months postoperative, she underwent 7 lengthening procedures with an average achievement of 10mm per session and resulting in a total expansion of 69.9mm. Her current status is CDF with a limb-length discrepancy of 19mm (left right). She is scheduled to be lengthened. Patient 2. Patient 2 was 9 years old at the time of surgery and 13 at the time of follow-up. He was diagnosed with osteosarcoma of the right proximal tibia and underwent prox- Fig 2. Patient 2 performing the sit-to-stand task. Reflective markers were placed over the patient s acromion, greater trochanter, lateral joint line of the knee, and lateral malleolus bilaterally.

4 1042 OUTCOMES USING AN EXPANDABLE PROSTHESIS, Beebe Fig 3. Patient 2 performing the gait task. Participants were asked to walk on a matrix of pressure sensors while temporal-spatial parameters were recorded. imal tibia replacement in March He completed 2 expansions beginning 10 months postoperative at an average of 6.2mm per expansion and for a total lengthening of 12.3mm. The patient underwent functional evaluation at 42 months postoperative and is currently CDF with a limb-length discrepancy of 10mm (right left). Patient 3. Patient 3 was 10 years old at the time of surgery and 11 at the time of follow-up. She was diagnosed with osteosarcoma of the left distal femur and underwent distal femur replacement in September She has not yet completed any expansions secondary to cardiomyopathy limiting sedation. The patient underwent functional evaluation at 15 months postoperatively and is CDF with a limb-length discrepancy of 20mm (right left). Patient 4. Patient 4 was 11 years old at the time of surgery and 14 at the time of functional evaluation. She underwent total femur replacement in April 2004 for Ewing sarcoma of the left distal femur and a skip lesion in the left proximal femur and began lengthening procedures at 10 months postoperatively. Until the time of functional evaluation at 33 months postoperative, the patient had undergone 5 expansions at an average expansion of 8.6mm per session. She has achieved a total lengthening of 42.9mm and is currently CDF with a limblength discrepancy of 8mm (right left). RESULTS Follow-up averaged 31.5 months (range, 15 42mo). Of the original 4 prostheses, 2 are undergoing active expansion. A total of 14 noninvasive expansions have been successfully performed, with an average length of 8.3mm achieved during each expansion. The average total expansion length was 41.7mm. No expansion complications were noted. For detailed data values, see table 1. Medical Outcomes Study 36-Item Short-Form Health Survey, Version 2 The mean scores on the PCS and MCS components of the SF-36 were 40 and 57, respectively (SDs 1.0 and 0.7 from the national mean) (table 2). Musculoskeletal Tumor Society The mean MSTS score was 78%, with average component scores of 4.8 for pain, 3.0 for Function, 4.3 for Emotional Acceptance, 4.3 for Supports, 4.3 for Walking Ability, and 3.0 for Gait (total 23.5 out of 30) (see table 2). Range of Motion and Strength Knee range of motion. Knee flexion ranged from 68 to 105 in the operated limb and 130 to 144 in the nonoperated limb (norm 130 ). Knee extension ranged from 2 (hyperextension) to 5 (flexion contracture) in the operated limb and from 2 to 8 in the nonoperated limb (norm 0 ). Knee strength. Flexion strength ranged from 4.3 to 11.65N in the operated knee and from 8.45 to 17.2N in the nonoperated knee. Knee extension strength ranged from 3.6 to 16.0N in the operated knee and from 12.1 to 27.3N in the nonoperated knee. Comparisons of knee strength in the nonoperated limb to normalized values were found to be 74% for flexion and 63% for extension. Hip range of motion. Hip ROM in the operated limb was slightly decreased in relation to the nonoperated limb. ROM in the operated hip was 96 to 111 during flexion (norm 125 ), 0 to 20 during extension (norm 15 ), and 35 to 52 during abduction (norm 45 ). ROM in the nonoperated hip was 105 to 122 during flexion, 10 to 18 during extension, and 30 to 62 during abduction. Table 1: Patient Expansion Details Patient 1 Patient 2 Patient 3 Patient 4 Mean Endoprosthetic replacement Distal femur Proximal tibia Distal femur Total femur NA Initial LLD (mm) Number of expansions 7 2 NL Total expansion (mm) NL Mean expansion per lengthening (mm) NL Abbreviations: LLD, limb-length discrepancy; NA, not applicable.

5 OUTCOMES USING AN EXPANDABLE PROSTHESIS, Beebe 1043 Table 2: SF-36 and MSTS Scores by Patient Scores Patient 1 Patient 2 Patient 3 Patient 4 Mean SF-36: PCS score SD from national mean SF-36: MCS score SD from national mean MSTS raw score % Score Hip strength. Hip flexion strength ranged from 3.8 to 15.25N in the operated limb and from 7.4 to 24.7N in the nonoperated limb. Hip extension strength ranged from 5.95 to 15.4N in the operated limb and from to 24.6N in the nonoperated limb. Abduction strength ranged from 0 to 28.25N in the operated limb and from 9.2 to 34.05N in the nonoperated limb. Comparisons of hip strength in the nonoperated limb to normalized values were 35% for flexion and 13% for extension. Sit-to-Stand Analysis of the sit-to-stand movement showed that maximum hip flexion ranged from 103 to 113 in the operated limb and from 103 to 106 in the nonoperated limb. Maximum hip extension was greatest in patient 2 whose primary surgical site was not the femur. Total joint excursion at the hip was smallest in patient 4, the sole patient whose proximal femur was replaced. Maximum knee flexion ranged from 60 to 91 in the operated limb and from 48 to 74 in the nonoperated limb. Maximum knee extension ranged from 1 to 6 in the operated limb and from 1 to 7 in the nonoperated limb. Peak hip flexion velocity ranged from 73 to 126 /s in the operated limb and from 64 to 120 /s in the nonoperated limb (norm /s). Peak hip extension velocity ranged from 157 to 208 /s in the operated limb and from 155 to 187 /s in the nonoperated limb (norm /s). Peak knee extension velocity ranged from 102 to 135 /s in the operated limb and from 84 to 135 /s in the nonoperated limb (norm /s). A measurement of ground reaction forces showed that peak vertical force was decreased in the operated limb, ranging from 225 to 336N versus 193 to 476N in the nonoperated limb. COM momentum, a comparison of horizontal relative to vertical momentum during the rising phase, averaged 86.8%, indicating a greater use of horizontal momentum during this phase of the sit-to-stand transition. The total horizontal COM displacement was 138% of normal. Movement time, or the time to complete the sit-to-stand maneuver from the onset of trunk motion while seated to the cessation of trunk motion in the upright stance, averaged 77.8% of normal (norm 1.63s). 32 Graphic analyses of the sit-to-stand task produced by the Peak Motus software confirmed that patients reached a maximum angular displacement later than healthy people and that the peak forward flexion and angular velocity of hip extension values were higher (fig 4). Greater force was placed on the nonoperated limb as compared with the affected limb, and graphic measures showed increased fluctuation in hip angle displacement and velocity during sit-to-stand transition (see figs 4 and 5). Gait Several aspects of gait functioning were decreased, including gait velocity (range, cm/s; norm 128cm/s), Fig 4. Hip angle displacement in the operated (red) and nonoperated (blue) limbs in a healthy person (A) and a patient (B). Fig 5. Hip angular velocities in the operated (red) and nonoperated (blue) limbs in a healthy person (A) and a patient (B).

6 1044 OUTCOMES USING AN EXPANDABLE PROSTHESIS, Beebe stride length (range, cm in the affected limb; cm in the unaffected limb; norm 128cm), and cadence (range, steps/min; norm 117 steps/min). Double-limb support was increased in all patients (range, % GC in the operated limb, % GC in the nonoperated limb; norm 20% GC), and stance phase was increased in all patients in the nonoperated limb (range, % GC; norm 60% GC) but only in half of the patients in the operated limb (range, % GC, norm 60% GC). Swing phase, step length, and step time were greater in the operated limb relative to the nonoperated limb. Subjective assessment of gait patterns revealed the following findings. Patient 1 showed lateral displacement of the trunk, especially evident when standing on the operated limb to extend the nonoperated limb. Trunk displacement was toward the side, ipsilateral to the affected limb. Additionally, the nonoperated limb lacked flexion during swing stance, creating a functionally longer limb by not shortening (ie, flexing) during gait. Patient 2 also exhibited a compensatory trunk swing, leaning toward the nonoperated side. Patient 3 displayed a classic right Trendelenburg pattern, with dropping of the left hip (ipsilateral to endoprosthetic replacement) during loading of the nonoperated leg. Although notable lateral displacement was not shown, patient 3 did exhibit a vaulting pattern as well as a stiff-knee strategy during loading of the operated leg. The operated knee showed very minimal flexion. Patient 4 showed lateral displacement toward the operated side during loading of the operated leg and ipsilateral trunk and advancement of the nonoperated limb. The operated limb exhibited diminished ROM as compared with the contralateral limb, and a substantial degree of momentum was used to propel the body forward. There was apparent weakness on her operated side, with a hip that pulled the trunk down ipsilaterally. A Trendelenburg was also observed on the nonoperated but not on the operated side (when loading the right hip, the left hip dropped). DISCUSSION Medical Outcomes Study 36-Item Short-Form Health Survey, Version 2 The scores achieved by our patients on the MCS subscale indicate adequate psychologic coping. However, the PCS component lagged behind at nearly 1 SD below the national mean, indicating that, despite persisting physical deficits, our patients have coped well while enduring surgical intervention and subsequent treatment. Musculoskeletal Tumor Society Subscale scores on the MSTS show that despite trouble with gait and overall functioning, our patients had low pain and high emotional acceptance. Walking ability was scored highly, and supports were only used by patient 3. It is important to note that patient 3 s total MSTS score of 56.7% deviated largely from that of the other 3 patients (range, %) and was thus likely to have an effect on the group s average MSTS score of 78.3%. Patient 3, whose outcome is compromised by severe cardiomyopathy, scored lower than the rest of the other group in the subscales of Emotional Acceptance, Supports, and Walking Ability. Overall, our patients scored highest on Pain (indicating very low levels of pain), slightly lower on Emotional, Supports, and Walking (indicating minor impairment in these areas), and lowest on Function and Gait (indicating moderate impairment in these areas). Although the goal of any treatment is to eradicate pain completely and restore complete functionality, we believe that these outcomes are highly satisfactory. Although the MSTS scoring system was implemented to promote standard, categorized evaluations, the system has several weaknesses that leave it incomplete as a solo analysis of patient outcome. In addition to the likelihood of interobserver variability, the system s attempt to encompass a wide variety of case types introduces the possibility that important information regarding different resections, reconstructions, and anatomic sites is not accounted for adequately. 17 Previous studies 19,21 have shown that MSTS scores often correlate poorly with actual measures of functioning. In addition, an investigation by Rompen et al 45 found that walking velocity was the only objective parameter positively correlated to the Gait component of the MSTS score when analyzed along with other measures including step length and swing phase. Given these noted discrepancies, it seems prudent to consider the MSTS scoring system in conjunction with other modes of evaluation. Range of Motion and Strength Significant decreases in strength and flexion were expected at the knee joint, which was the primary tumor and resection location; however, patients also showed decreased strength at the hip. Additionally, although decreases in ROM and strength were expected in the operated extremity, our results show that patients actually display bilateral decreases in strength at both the hip and knee. This is important because patients recovering from lower-extremity limb-salvage surgery tend to increase weight bearing on the contralateral leg during rehabilitation. Although some amount of general deconditioning is expected, the degree of weakness was unexpected and emphasizes the need for therapy regimens that focus on overall conditioning and strengthening at the hip and knee bilaterally. Our data suggest that beyond the expected and observed disability in and around the knee, patients who undergo resection and reconstruction also tend to have marked weakness in the hips that must not be overlooked. Furthermore, the bilateral nature of weakness at the hip and knee joints warrants careful attention during planning and execution of the patient s physical therapy. Sit-to-Stand Motion A dramatic difference in peak ground reaction force was observed between the operated and nonoperated lower limbs, indicating that our patients rely more heavily on the latter to steady themselves during the task. In healthy people, the first phase of the sit-to-stand motion involves the horizontal generation of momentum from the body s COM, accompanied by a slight increase in hip flexion. This occurs while the subject remains seated. The horizontal momentum is then transferred into a vertical momentum by a sequence of hip flexion, trunk flexion, head extension, and ankle dorsiflexion and leads to a progressive change from hip flexion to hip extension as vertical momentum of COM is gained. In addition to the strength and coordination needed to generate this horizontal momentum, there must also be sufficient control of the trunk and hip muscles to slow forward motion and prevent falling forward as well as adequate lowerextremity joint integrity and strength for extension during rising. 46 Although children and adults typically show comparable form and sequence during the sit-to-stand transition, 43 younger populations nevertheless tend to have a smaller (39%) horizontal-to-vertical ratio of COM momentum compared with the elderly (97%), 47 indicating that the latter require greater momentum to propel themselves forward during the initial stage.

7 OUTCOMES USING AN EXPANDABLE PROSTHESIS, Beebe 1045 This may be caused by weakness at the hips and knees, forcing them to use momentum to propel the body forward and up rather than rely on the strength of the quadriceps and hip extensors. The average COM momentum ratio in our patients was 87%, which more closely approximates values seen in the elderly despite the mean patient age of 12.5 at the time of functional evaluation. Additionally, our patients had greater excursion velocities during the sit-to-stand transition, attaining maximum angular displacement sooner than would normally be expected. This suggests that they do indeed rely on the generation of horizontal momentum to swing themselves into a vertical standing position. Accordingly, our patients are likely to be weaker at the hips and knees, although other possible variables to be considered include the amount of muscle resected, comorbidities such as cardiomyopathy from chemotherapy, and surgical complications including wound necrosis and infection. Thus, it is necessary to emphasize the importance of strengthening the quadriceps and hip extensor muscles in these patients, with a special focus on improving the ability to perform basic essential tasks such as rising from a chair. There is evidence that changes in load/capacity ratio in healthy people (ie, the ratio between body mass and the force generated by muscle strength) can produce similar effects, mimicking the reduced muscle capacity typically found in the elderly or patients with decreased muscle strength. A study by Savelberg et al 48 found that increasing load/capacity to 45% produced a change in sit-to-stand strategy, as reflected by an increase of more than 10 during maximum trunk flexion, a disproportionate increase in net hip extension, and a delayed peak of net knee extension moment. Another noteworthy trend is the unpredictable nature of the hip angular displacement and angular velocity values across time. Although graphic analysis of a healthy person performing the task shows a smooth curve, that of our patients shows much greater variability. This may be a result of unstable movement, with variations in force occurring as weight distribution fluctuates and is transferred to the lower limbs and from 1 limb to the other. However, other factors to keep in consideration include weakness from the surgery or amount of muscle resected as well as the possibility that movement patterns have not yet fully been established in these young patients. Additionally, previous data have indicated that bilateral symmetry of lower-extremity joint movements during the sit-to-stand task cannot be assumed, 49 and the lack of refined motor control in children has also been implicated in causing poor movement consistency when transitioning from sitting to standing. 43 Gait The increased stance phase in the nonoperated limb, increased swing phase in the operated limb, and increased double-limb support confirm that our patients entrust their nonoperated limb with more weight bearing. Decreased stride length in the operated limb indicates smaller steps with that limb, a strategy that may be implemented to decrease force to that leg. The difficulty of the task is further made apparent by decreased cadence. Increased time spent in double-limb support and decreased gait velocity seem to indicate weakness of the hip abductors because strength in these muscles has been correlated with the net energy cost and symmetry of walking, whereas the degree of asymmetry of single-limb support time has been negatively correlated with gait velocity. 50 Evidence also shows that the extent and exact location of quadriceps removal specifically influences patient gait functionality. A study by Benedetti et al 51 found that, compared with patients who had the vastus medialis and vastus intermedius removed, patients with vastus lateralis and vastus intermedius had better gait performance, were less likely to develop a stiff knee gait, and had a more normal flexion-extension knee joint pattern. The Trendelenburg and vaulting gait deviations noted in our patients are most likely because of hip weakness and a resulting lack of stance control. Vaulting, an accessory vertical displacement of pushing off on 1 leg to advance the other, is a commonly used mechanism to account for limb-length discrepancy. Patient 3 was the only member of the group to show vaulting; in addition, a distinct bilateral weakness was present, accompanied by a stiff knee gait most likely caused by weakened muscle. Flexion in the operated knee was minimal except during the terminal swing phase, resulting in a functionally longer leg that is likely to explain the vaulting pattern. Rather than the lateral swing motion noted in the rest of the group, patient 3 compensates with upward displacement that allows sufficient room for the operated limb to swing through. It is possible that this pattern is used because of slightly more strength in the contralateral leg than may be seen in the other patients, leading to an ability to maintain increased force. Weakness of 1 hip as observed throughout our patient group can be a result of destabilization. With the inability to stabilize 1 hip to bend the contralateral knee, stability is attempted by swinging the contralateral knee out. All patients compensation methods were proximal except for patient 2, the only patient to have a proximal tibial replacement. CONCLUSIONS Based on our observations from this study, reconstruction with a noninvasive expandable endoprosthesis produces satisfactory functional outcomes in pediatric patients with primary tumors of the bone. Furthermore, because no complications were encountered with the expansion procedures, it appears that an endoprosthesis such as the Repiphysis can successfully minimize the risks associated with traditional lengthening techniques, without contributing additional negative consequences arising directly from the lengthening procedure. However, a main limitation to our study was its small sample size, making it difficult to generalize our results toward a larger and more diverse group of patients. The series of assessments performed in this study provided a broad understanding of our patients physical and emotional health after a period of rehabilitation, identifying areas of fair functioning while pointing out others that require more attention to reach satisfactory outcome. A surprising but important finding in our patient group was the persisting hip weakness in both operated and nonoperated limbs, up to 42 months after resection around the knee. This indicates that continued rehabilitation programs emphasizing hip strengthening should be considered for these patients, even years after surgery. The ability to perform limb-salvage surgery on pediatric patients while minimizing complications and surgical adverse events is important in attempting to provide as natural a childhood as possible. We determined that our patients continued to perform well not only physically but without evidence of emotional or psychologic suffering, a critical component in the goal to enhance QOL for these young patients. In light of our findings, we believe that the knowledge gained from this study provides valuable insight with regard to the next steps in furthering functional outcome in this patient population. The variety of measures used in this study provided information allowing further focus on the areas most in need of

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