Biomedical subjects

R Alexander Mohr

Publications and source records attributed to R Alexander Mohr.

6 recordsLinked to original sources

PubMed2005

Revision total knee arthroplasty with modular components inserted with metaphyseal cement and stems without cement.

The clinical and radiographic outcomes of 50 consecutive revision total knee arthroplasties in 47 patients, placed with metaphyseal cemented femoral and tibial components with press-fit cementless stems, were reviewed at 36-month average follow-up. Revision was performed for aseptic loosening (11/50), infection (17/50), periprosthetic fracture (8/50), component failure (6/50), instability (6/50), and malalignment (2/50). The press-fit cementless stems were 80 to 160 mm in length and tightly contacted the endosteum of the metadiaphyseal areas. Four (9%) knees were re-revised for infection, zero for aseptic loosening. The average modified Hospital for Special Surgery knee score improved from 49 to 87. One patient (2%) reported thigh pain, and 1 reported leg pain. Metaphyseal cemented revision total knee components with press-fit cementless femoral and tibial stems were not associated with significant thigh and leg pain.

Adult↗

PubMed2003

Anterior thoracolumbar instrumentation: stiffness and load sharing characteristics of plate and rod systems.

STUDY DESIGN: An in vitro biomechanical study using a thoracolumbar corpectomy model to compare load sharing capabilities and stiffnesses of six different anterior instrumentation systems (three rod styles and three plate styles) for stabilizing the thoracic and lumbar spine. OBJECTIVES: To evaluate the axial load sharing capabilities of the instrumentation in a thoracolumbar corpectomy model and to measure the bending stiffness of the anterior instrumentation systems for the axes of flexion-extension, lateral bending, and axial rotation with and without an anterior column graft in place. SUMMARY OF BACKGROUND DATA: Prior publications have analyzed biomechanical characteristics of many spinal instrumentation systems. These reports have compared anterior instrumentation systems with posterior instrumentation systems, in situ fusion techniques, intervertebral spacers, structural allograft and instrumentation, and combined anterior and posterior instrumentation. Other reports have published data on the biomechanical characteristics of typical anterior and posterior spinal instrumentation systems. However, there are no published reports that specifically compare the characteristics of anterior plate-style with anterior rod-style systems, or examining load sharing capabilities. METHODS: Six constructs of each of six instrumentation systems were mounted on simulated vertebral bodies. A custom four-axis spine simulator was used to apply independent flexion-extension, lateral bending, and axial rotation moments as well as axial compressive loads. Axial load sharing was measured through a range of applied axial loads from 50 N to 500 N with rotational moments maintained at 0 Nm. The bending stiffness of each construct was calculated in response to +/-5.0 Nm moments about each axis of rotation with a 50 N compressive axial load with a full-length corpectomy graft in place, simulating reconstruction of the anterior column, and with no graft in place, simulating catastrophic graft failure. Statistical significance was determined using an analysis of variance and Fisher PLSD post hoc test with an alpha <or= 0.05. RESULTS: Load sharing results ranged from 63% to 89%. There was an inverse relationship between load sharing and stiffness. No correlation was found between load sharing and implant style (rod vs. plate). With the graft in place, stiffness result varied by instrumentation system rather than by plate/rod style. Without the graft, the stiffness of the constructs decreased approximately one-third in flexion-extension, two-thirds in lateral bending, and one-fifth in axial rotation, underlying the importance of the graft in overall construct stiffness. CONCLUSIONS: For both load sharing and stiffness, there is more influence from the design of the instrumentation system, than whether it is a plate or rod style system. The graft contributed to overall construct stiffness, particularly in lateral bending.

Animals↗

PubMed2003

Tibial component fixation with cement: full- versus surface-cementation techniques.

Despite excellent outcomes with cemented tibial components in total knee arthroplasty, it still is debated whether the tibial stem should be cemented and what the optimal tibial stem design should be. Proponents of full cementation of the tibial stem and component state that better short-term and long-term component fixation is achieved when full cementation is used. Advocates for surface cementation contend that sufficient implant stability is achieved without the increased bone loss that occurs at revision and the stress shielding thought to be linked with cemented stems. This biomechanical cadaver study compared initial fixation and cement penetration depth in fully cemented versus surface cemented tibial trays with two different stem geometries (cruciate and I-beam) and compared two stem designs (cruciate and I-beam) fixed with surface cementation. Under an eccentric load, simulating three times body weight for 6000 cycles, there seems to be no difference in the micromotion of either tibial component implanted with surface or full cementation. Additionally, no difference in the average depth of cement penetration was detected between fixation techniques or stem types. The initial fixation stability of the surface cement technique seems correlated to the depth of cement penetration into proximal tibial surface. The current data support other studies which indicate that stability of surface-cemented tibial components may be related to the depth of cement penetration.

Adult↗

PubMed2003

Rotational stability of a modified step-cut for use in intercalary allografts.

BACKGROUND: Intercalary allografts are used for the reconstruction of major skeletal defects. Step-cuts help to provide rotational stability when intramedullary fixation is used. A modified step-cut is proposed to reduce rotation at the interface. This study compares the rotational stability of conventional and modified step-cuts. METHODS: In Phase I, seven pairs of human cadaveric femora were divided into a conventional step-cut group (left femora) and a modified step-cut group (right femora). All femora were cut transversely at the mid-diaphysis. In the conventional group, a 1-cm step-cut was created in the exact midsagittal plane in both the proximal and distal segments. In the modified group, a 1-cm step-cut was created in the parasagittal plane, leaving 2 mm of additional bone on both the proximal and the distal fragment. Phase II was identical except that in the modified step-cut group only 1 mm of additional bone was left. Smooth femoral nails were then placed after standard reaming. Specimens were tested by fixing the proximal segment and applying +/-2 N-m (17.7 in-lb) of torque to the distal segments with ten oscillation cycles. Maximum rotation was measured. The data were analyzed with the paired Student t test. RESULTS: The average rotation in Phase I was 23.3 degrees for the conventional step-cut group and 3.0 degrees for the 2-mm modified step-cut group; the difference was significant (p < 0.001). Four femora sustained an incomplete fracture during nail insertion. The average rotation in Phase II was 20.6 degrees for the conventional step-cut group and 0.5 degrees for the 1-mm modified step-cut group without any fractures; the difference was significant (p < 0.001). CONCLUSIONS: Step-cut modification that leaves more bone in the sagittal plane provides rigid fixation and significantly more stability than the conventional step-cut technique.

Biomechanical Phenomena↗

PubMed2001

Dynamic cervical plates: biomechanical evaluation of load sharing and stiffness.

STUDY DESIGN: An in vitro biomechanical study using a simulated cervical corpectomy model to compare the load-sharing properties and stiffnesses of two static and two dynamic cervical plates. OBJECTIVES: To evaluate the load-sharing properties of the instrumentation with a full-length graft and with 10% graft subsidence and to measure the stiffness of the instrumentation systems about the axes of flexion-extension, lateral bending, and axial torsion under these same conditions. SUMMARY OF BACKGROUND DATA: No published reports comparing conventional and dynamic cervical plates exist. METHODS: Six specimens of each of the four plate types were mounted on ultra-high molecular weight polyethylene-simulated vertebral bodies. A custom four-axis spine simulator applied pure flexion-extension, lateral bending, and axial torsion moments under a constant 50 N axial compressive load. Load sharing was calculated through a range of applied axial loads up to 120 N. The stiffness of each construct was calculated in response to +/-2.5 Nm moments about each axis of rotation with a full-length graft, a 10% shortened graft, and no graft. ANOVA and Fisher's post hoc test were used to determine statistical significance (alpha < or = 0.05). RESULTS: The two locked cervical plates (CSLP and Orion) and the ABC dynamic plate were similar in flexion-extension, lateral bending, and torsional stiffness. The DOC dynamic plate was consistently less stiff. The Orion plate load shared significantly less than the other three plates with a full graft. Both the ABC and the DOC plates were able to load share with a shortened graft, whereas the conventional plates were not. CONCLUSIONS: All plates tested effectively load share with a full-length graft, whereas the two dynamic cervical plates tested load share more effectively than the locked plates with simulated graft subsidence. The effect of dynamization on stiffness is dependent on plate design.

Bone Plates↗

PubMedDate not supplied

Gait analysis of stapling for genu valgum.

Many authors have advocated stapling or epiphysiodesis of the distal medial femur as a means of correcting genu valgum. However, in the literature, aside from clinical improvement (appearance, pain, function), objective evidence of kinetic and kinematic improvement is lacking. Therefore, the authors undertook a prospective gait analysis evaluation of a series of patients treated for genu valgum, comparing pre- and postsurgical measurements to document the benefits of normalizing the mechanical axis. These results indicate that after surgery knee and hip angles and knee moments were returned to within the normal range for a similarly aged control group.

Adolescent↗