SPECT/CT delineation of proximal screw loosening and distal pseudoarthrosis following total hip replacement

By Christian Waldherr, MD; Prof. Hertel, MD; and Martin Sonnenschein, MD
Data courtesy of Klinik Engeried, Lindenhofgruppe, Bern, Switzerland 

A 74-year-old female underwent right side total hip replacement in 2001. In 2006, the hip prosthesis was replaced due to low-grade infection. A periprosthetic fracture was delineated on routine radiography and CT in 2014. A pathological fracture was suspected due to exaggerated bone loss in fracture area, but there was no evidence of tumor or histology of other bony pathology, such as Paget’s disease. The patient complained of progressive pain and limitation of movement, as well as instability of the hip joint during walking. Patient underwent a ^99mTc MDP bone SPECT/CT to assess the complex post-arthroplasty situation.SPECT/CT of the hip joint and proximal femur using a single 360-degree acquisition was performedon a Symbia Intevo™ scanner 3 hours following an IV injection of 600 MBq (16.22 mCi) ^99mTc MDP. Planar whole-body acquisition preceded SPECT/CT acquisition. Following a thin-slice diagnostic CT (130 kV, 72 eff mAs 16 x 0.6 mm collimation), SPECT was acquired with 32 stops per detector at 20 sec/stop. SPECT reconstruction was performed using 3DOSEM (Flash 3D). Reconstructed SPECT data was fused with the CT for a final evaluation.

The maximum intensity projection (MIP) images of SPECT and CT as well as volume rendering of the fused SPECT/CT data (Figures 1 and 2) demonstrate the skeletal stress related to the loosening of the proximal screws, especially the upper screws extruded from the bone as shown on CT. The loosening of the last fixed proximal screw attached to the distal end of the shaft of the femoral component of the hip prosthesis is probably secondary to the shear stress following extrusion of the upper screws, which also reflects associated severe periprosthetic osteoporosis.

The periprosthetic fracture adjacentto the distal end of the femoral component of the hip prosthesis is associated with avascularity of the bony segment with distal pseudoarthrosis and reactive hypermetabolism, as evident in the SPECT/CT images.

Figures 3 through 7 delineate the degree of loosening of the proximal screws with extrusion of the uppermost screws and resulting shear stressat the level of the proximal screws and plate, especially the last fixed proximal screw, reflecting the abnormal prosthetic motion secondary to loosening. CT also shows severe periprosthetic osteoporosis. The bone around the lower part of the shaft of the femoral component of the prosthesis appears completely avascular with sclerosis and multiple periprosthetic fractures with absence of tracer uptake, indicating necrosis.

This case illustrates the value of combining SPECT and CT for assessment of such complex post-prosthetic surgery situations. CT in isolation demonstrates the extruded proximal screws and periprosthetic fractures while addition of ^99mTc MDP SPECT highlights the degree of shear stress at the proximal screws and delineates the avascular segment of the femoral shaft around the lower end of the prosthetic shaft, as well as the pseudoarthrosis between the avascular segment and the normal lower femoral shaft. The acetabular component shows no loosening. A comprehensive under-standing of the morphological and functional status of the femoral prosthetic component, the screws and plates, as well as that of the impacted femoral bone, including delineation of the avascular segment and pseudoarthrosis helps define the surgical management of this patient, especially the amount of avascular bone to be resected and the degree of bone graft placement, along with replacement of the femoral prosthesis.