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Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_5 | Pages 111 - 111
1 Mar 2017
Reynolds R Walker P Buza J Borukhov I
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INTRODUCTION

Understanding the biomechanics of the anatomical knee is vital to innovations in implant design and surgical procedures. The anterior – posterior (AP) laxity is of particular importance in terms of functional outcomes. Most of the data on stability has been obtained on the unloaded knee, which does not relate to functional knee behavior. However, some studies have shown that AP laxity decreases under compression (1) (2). This implies that while the ligaments are the primary stabilizers under low loads, other mechanisms come into play in the loaded knee. It is hypothesized this decreased laxity with compressive loads is due to the following: the meniscus, which will restrain the femur in all directions; the cartilage, which will require energy as the femur displaces across the tibial surface in a plowing fashion; and the upwards slope of the anterior medial tibial plateau, which stabilizes the knee by a gravity mechanism. It is also hypothesized that the ACL will be the primary restraint for anterior tibial translation.

METHODS

A test rig was designed where shear and compressive forces could be applied and the AP and vertical displacements measured (Figure 1). The AP motion was controlled by the air bearings and motor, allowing for the accurate application of the shear force. Position and force data were measured using load cells, potentiometers, and a linear variable differential transducer.

Five knee specimens less than 60 years old and without osteoarthritis (OA), were evaluated at compressive loads of 0, 250, 500, 750 N, with the knee at 15° flexion. Three cycles of shear force at ±100 N constituted a test. The intact knee was tested, followed by testing after each of the following resections: LCL, MCL, PCL, ACL, medial meniscus, and lateral meniscus.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_6 | Pages 75 - 75
1 Mar 2017
Walker P Borukhov I Bosco J Reynolds R
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INTRODUCTION

Most total knees today are CR or PS, with lateral and medial condyles similar in shape. There is excellent durability, but a shortfall in functional outcomes compared with normals, evidenced by abnormal contact points and gait kinematics, and paradoxical sliding. However unicondylar, medial pivot, or bicruciate retaining, are preferred by patients, ascribed to AP stability or retention of anatomic structures (Pritchett; Zuiderbaan). Recently, Guided Motion knees have been shown to more closely reproduce anatomic kinematics (Walker; Willing; Amiri; Lin; Zumbrunn). As a design approach we proposed Design Criteria: reproduce the function of each anatomic stabilizing structure with bearing surfaces on the lateral and medial sides and intercondylar; resected cruciates because this is surgically preferred; avoid a cam-post because of central femur bone removal, soft tissue entrapment, noises, and damage (Pritchett; Nunley). Our hypothesis was that these criteria could produce a Guided Motion design with normal kinematics.

METHODS & MATERIALS

Numerous studies on stability and laxity showed the ACL was essential to controlling posterior femoral displacement on the tibia whether the knee was loaded or unloaded. Under load, the anterior upwards slope of the medial tibial plateau prevented anterior displacement (Griffen; Freeman; Pinskerova; Reynolds). The posterior cruciate and the downward lateral tibial slope produced lateral rollback in flexion. The Replica Guided Motion knee had 3 bearings (Fig 1). The lateral side was shallow and sloped posteriorly, with a posterior lip to prevent excess displacement. The medial anterior tibial and femoral slopes were increased as in the anatomic knee. In the intercondylar region, a saddle bearing replaced ACL function by controlling posterior femoral displacement. For testing, a typical PS design was used as comparison. A Knee Test Machine (Fig 2) flexed the knee, and applied axial compression, shear and torque to represent a range of functions. Bone shapes were reproduced by 3D printing and collaterals by elastomeric bands. Motion was recorded with a digital camera, and Geomagic to process data.


Orthopaedic Proceedings
Vol. 87-B, Issue SUPP_III | Pages 320 - 321
1 Sep 2005
Tolo V Skaggs D Storer S Friend L Chen J Reynolds R
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Introduction and Aims: Surgical correction of pelvic obliquity is an important component of spinal instrumentation for neuromuscular scoliosis, though instrumentation to the pelvis has high reported complication rates. This study evaluates the results of pelvic fixation during surgical correction of neuromuscular scoliosis in a consecutive series of 62 children and adolescents.

Method: A retrospective chart and radiographic review of 62 consecutive patients treated with spinal fusions to the pelvis as treatment for neuromuscular scoliosis was performed. Follow-up ranged from two to seven years. Diagnoses included cerebral palsy (36 patients), muscular dystrophy (16 patients), myelomeningocele (three patients), spinal muscular atrophy (three patients) and other disorders (four patients). Mean age at surgery was 13.5 years. Pelvic fixation techniques used included Luque-Galveston or iliosacral screw fixation. Correction of deformity in each patient was assessed with Cobb angle measurements of scoliosis, thoracic kyphosis, and lumbar lordosis. Pelvic obliquity and coronal decompensation was also assessed.

Results: The Luque-Galveston spinal instrumentation technique was used in 54 patients and iliosacral screw fixation was used in eight patients. Seventeen patients had an additional anterior release and fusion without instrumentation. The mean Cobb angle measured 73 degrees pre-operatively and 31 degrees (mean correction 59%) post-operatively. The mean Cobb angle on latest follow-up was 33 degrees (loss of correction 12%). Thoracic kyphosis remained essentially unchanged, as did lumbar lordosis (56 pre-op and 61 on follow-up). Pelvic obliquity corrected from a mean of 16 degrees pre-operatively to eight degrees on most recent follow-up. Mean pre-operative coronal decompensation measured 135mm, and follow-up decompensation measured 46mm. Eleven patients with Galveston fixation exhibited the ‘windshield-wiper’ sign, with a radiolucency of 2mm or more, though most were asymptomatic. Wound infection was observed in 6% (3/54) of the patients who underwent Galveston instrumentation and 50% (4/8) who had iliosacral screws. In patients treated with Galveston fixation, three had symptomatic prominant hardware and one had hardware breakage for an overall mechanical failure rate of 7% (4/54). In contrast, two patients with iliosacral screws had construct breakage and pseudoarthrosis for a mechanical failure rate of 25% (2/8), though the numbers in the iliosacral screw group are small.

Conclusions: In this series, Galveston pelvic fixation during spinal instrumentation treatment of neuromuscular scoliosis was associated with satisfactory results and with less complications than generally reported in the literature. This technique is recommended as the preferred method for pelvic fixation in severe neuromuscular scoliosis associated with pelvic obliquity.