Gösta Ullmark

Impacted corticocancellous allografts: Recoil and strength

Bone allografts were morcellized using 2 different milling machines (Tracer milling machine and Howex milling machine) producing bone chips with different ranges of sizes. In an in vitro model, each type of bone-graft was impacted with 2 different impaction forces. As the impaction force was released, there was a substantial and rapid recoil of the graft bed. More recoil was seen with the smaller chips and with the harder impacted graft beds. Most of the recoil occurred within the first 10 seconds. Using another similar in vitro model, the same 2 kinds of chips were impacted with 2 different impaction forces, and the subsidence of the 4 different graft beds was measured. There was less subsidence with the bigger type of bone chips and less with the harder impacted graft beds.

Bone regeneration 6 years after impaction bone grafting A PET analysis

Background Impacted morselized bone allograft in revision total hip arthroplasty for prosthetic loosening has gained widespread clinical use during the last decades. The clinical results are good but little is known about the bone regeneration in the graft. Patients and methods 5 patients were revised with impaction of morselized frozen allograft and a cemented total hip arthroplasty (THA) due to loosening and osteolysis of a primary THA. We used positron emission tomography ([18F]-fluoride PET) to produce quantitative images of new bone formation in the allograft surrounding the femur stem 6 years after surgery. Results The 5 patients had previously been analyzed by [18F]-fluoride PET during the first year after surgery (Sörensen et al. 2003). During the first year, bone formation proceeded through the graft layer and reached the cement layer surrounding the femoral stems. The clinical and radiographic results were excellent at 6 years. PET analyses at 6 years showed that the bone metabolism was significantly reduced in most areas of the proximal femur, compared to the elevated activity during the first year after surgery, and also normalized compared to the contralateral healthy femur. Graft bone metabolism distal to the stem tip remained slightly increased. Small patchy areas of increased uptake remained along the proximal femoral stem, probably reflecting small volumes of fibrous healing. Interpretation The metabolism of the allografted bone had normalized compared to native bone, indicating full regeneration throughout the graft—and a good long-term prognosis for implant fixation.

Bone healing of severe acetabular defects after revision arthroplasty

Background and purpose Healing of acetabular bone grafts may be difficult to assess in conventional radiographs. We used PET to analyze healing of morselized bone allografts, impacted in large osteolytic acetabular defects at revision arthroplasty. Patients and methods 7 cases had a cup revision because of loosening, with repair of a segmental defect using a perforated, wide and thin plate. The osteolytic acetabulum was impacted with morselized bone allograft before cementing a polyethylene cup. [18F]-fluoride PET scans were used to monitor bone healing inside the graft bed 1 week, 4 weeks, and 12 months after surgery. The contralateral pelvic bone above the acetabulum was used as reference. A second group of 4 cases was analyzed for bone-forming activity in the state of mechanical loosening of an acetabular component of a THA. Results Preoperatively, the uptake was raised by 64% compared to the reference. 1 week after surgery it was increased by 77% in segmental regions, while the uptake was at the reference level in cavitary regions. After 4 months the uptake was increased by 91% in cavitary regions and by 117% in segmental regions. 1 year after surgery, the increase in uptake was 20% in both regions. All implants were stable on radiographs. InterpretationWe found PET to be a sensitive and useful method for evaluation of the spatial and temporal development of bone formation.

Osteonecrosis following resurfacing arthroplasty: A clinical positron emission tomography study of 14 cases

Background and purpose One of the main concerns regarding resurfacing arthroplasty is the viability of the remaining part of the femoral head, and the postoperative risk of a femoral neck fracture or collapse. In contrast to radiographic methods, positron emission tomography using the radiotracer [18F]-fluoride (Fluoride-PET) enables us to visualize the viability of bone in the remaining part of the head, despite the presence of the covering metal component. Patients and methods This is preliminary prospective study of 14 patients who underwent an ASR resurfacing arthroplasty. Apart from clinical and radiographic analyses, all patients were analyzed by PET scan 1 week, 4 months, and 1 year after surgery. Results 1 patient had a minor region of osteonecrosis on PET scan at 1 week and at 4 months. After 1 year, the necrosis had increased to include most of the head. 2 other patients, normal at 4 months, had developed equally large osteonecrosis at 1 year. A fourth patient had a minor osteonecrosis at 1 year. None of the patients had clinical symptoms, and the necrotic areas were not visible on plain radiographs. Conclusions We found Fluoride PET to be a sensitive and useful method for evaluation of bone metabolism at resurfacing arthroplasty. 3 of the 14 patients had developed osteonecrosis, involving most of the head at 1 year. The late onset of the phenomenon does not support the hypothesis of surgically damaged vascularity. The presence of this complication together with the lack of visibility on plain radiographs gives reason for concern.

Analysis of bone mineralization on uncemented femoral stems by [18F]-fluoride-PET: A randomized clinical study of 16 hips in 8 patients

Purpose We present the first study using fluoride-positron emission CT (F-PET/CT) to analyze mineralization of bone in the femur adjacent to uncemented stems following total hip arthroplasty (THA). We studied patients who were operated bilaterally for osteoarthritis with 2 different stems during the same surgical session. Patients and methods THA was performed bilaterally during the same surgical session in 8 patients with bilateral osteoarthritis of the hip. An SL-PLUS stem was inserted in one hip and a BetaCone stem was inserted in the contralateral hip, with randomization of side and sequence. A second group of 12 individuals with a normal healthy hip was used as reference for normal bone metabolism. Clinical and radiographic evaluation was performed preoperatively, postoperatively, and at 2 years. We used [18F]-fluoride-PET/CT to analyze bone mineralization adjacent to the stems 1 week, 4 months, and 12 months after surgery. We modified the Polar Map system to fit the upper femur for analysis and presentation of the PET results from 12 regions of interest adjacent to the whole stem. Results The clinical results were good at 2 years. By radiography, all stems were stable. At PET analyses 1 week after surgery, the activity was higher for the SL-PLUS group than for the BetaCone group. The activity was statistically significantly higher for both stems than the reference values at 4 months, and was most pronounced in the upper femur. At one year, the activity had declined more for the BC group than for the SL group. Interpretation The bone mineralization activity varied between different regions for the same stem and between different time periods for each group. F-PET/CT is a novel and valuable tool for analysis of bone mineralization patterns around uncemented femoral stems in detail. The combination of PET/CT analysis and the modified Polar Map system may provide a useful tool for future studies of metabolic bone responses to prosthetic implants.

Analysis of bone formation on porous and calcium phosphate-coated acetabular cups: a randomised clinical [18F]fluoride PET study.

We present a study using Fluoride-Positron Emission Tomography (F-PET/CT) to analyse new bone formation in periacetabular bone adjacent to press fit cups following THA. In 16 THA (8 patients) with bilateral hip osteoarthritis simultaneous bilateral total hip arthroplasty (THA) was performed, employing electrochemically applied calcium phosphate coated (HA) cups or porous-coated (PC) cups allocated at random to compare the two sides. A reference group of 13 individuals with a normal healthy hip was used to determine ‘normal’ bone metabolism. [18F]fluoride -PET/CT was used to analyze bone formation adjacent to the cups 1 week, 4 months and 12 months after surgery. Clinical and radiographic evaluation was performed preoperatively, postoperatively and at 2 years. Bone forming activity had a mean of 5.71, 4.69 and 3.47 SUV around the HA- and 5.04, 4.80 and 3.50 SUV around the PC-cups at 1 week, 4 months and 12 months respectively. Normal bone metabolism was 3.68 SUV. After 1 year activity had declined to normal levels for both groups. The clinical results were good in all cases. HA coating resulted in higher uptake indicating higher bone forming activity after 1 week. F-PET/CT is a valuable tool to analyse bone formation and secondary stabilisation of an acetabular cup.

PET scanning for evaluation of bone metabolism

Sir–With great interest we read the paper of Ullmark et al. on using positron emission tomography (PET) to assess bone metabolic activity (Ullmark et al. 2009). PET is an imaging tool that can provide functional rather than morphological information, and has thus attracted a lot of attention. The prerequisites for proper analysis and subsequent sufficient interpretation are very important, and most of these originate from the principles of PET. Unfortunately, Ullmark et al. do not address these important issues and, to avoid misinterpretations, we wish to highlight some of these important points. The principle of positron emission tomography PET scanning is based on a short-lived radioactive tracer, which decays by emitting a positron. The tracers can exist as single isotopes or be incorporated in larger molecules, depending on the application. This method suffers from low spatial resolution. Apart from single-positron emission tomography (SPECT), all PET systems utilize coincidence detection of the annihilation photons from positron decay. Since the paired gamma rays from the annihilation of the positron are anti-parallel, the detection of the gamma rays determines a line of response (LOR) along which the annihilation took place. Resolution is thus determined by the size/density of the detectors that are most often placed in a ring around the subject. However, the LOR is not completely linear and the gamma rays are released in a direction of 180° ± 0.5°. This physical phenomenon further limits the resolution. The spatial resolution of most clinical PET scanners is therefore only about 6–8 mm (Townsend 2004). Hence, the resulting images are known to be affected by partial-volume effects, which can cause small regions with high tracer uptake to be imaged as having an artificially low concentration and vice versa (Soret et al. 2007). When comparing regions of interest (ROIs), it is therefore important for the accuracy that the size is sufficiently high or that the tissue surrounding the ROIs has a comparable uptake of the tracer. For the reader to be able to interpret PET data, describing the size of the ROI is as important as the obvious adding of standard deviations to the results. The three basic analytical methods with or without blood sampling will not be addressed here. In orthopedic approaches, the use of the fluoride isotope [18F] is of particular interest because of its incorporation into the bone crystals. However, due to the rate of bone formation, [18F] is not incorporated into bone during a 1- to 2-hour scan. The fluoride ion exchanges with the hydroxyl groups in the hydroxyapatite crystal of bone (Ca10(PO4)6OH2) to form fluoroapatite (Ca10(PO4)6F2), and this can be interpreted as a 4-step event, which was originally described by Blau et al. as early as 1962—long before PET gained its clinical popularity and accessibility. The first 2 steps from the blood through extracellular space to the shell of bound water around exposed crystals are very rapid (minutes), step 2 being the irreversible step. Step 3 is the traveling of the tracer onto the crystal surface, and this is probably measured in hours. The final step of incorporation, step 4, may take days or weeks. Consequently, areas of high uptake result from processes that increase the exposed bone crystal surface and/or increase blood flow. Thus, areas of osteoclastic and osteoblatic activity are measured equally, and this is of crucial importance when interpreting these data sets (Blau et al. 1972). By directly translating SUV to “bone forming activity”, Ullmark et al. draw a dicey conclusion. This translation is apparently based on a study by Piert et al. correlating metabolic activity in the vertebral body from PET and histomorphometry in an iliac crest biopsy (Piert et al. 2001). However, that study investigated bone blood flow and metabolic rate in untreated bone, which most likely presents a steady state metabolic rate. This is obviously very different from new bone formation in an allograft-implanted area. The findings at 1 week postoperatively in Ullmarks study could very well be due to impaired blood flow in the impacted graft rather than being an assessment of metabolic rate per se (Berding et al. 1995, Piert et al. 1998). Furthermore, there is evidence that radiotracer uptake is not dependent on osteoblast number, but concentration of bone-forming minerals (Toegel et al. 2006). It is therefore important to notice that areas of osteoclastic and osteoblastic activity are labeled equally, and thus increased resorption would also result in increased tracer uptake without it necessarily resulting in a net increase in bone volume (Blau et al. 1972, Genant et al. 1974). Combined PET/CT and (most recently) PET/MRI scanners make it possible to correlate the functional data from PET with morphological images (Foldager et al. 2008). This has two advantages. First, it gives an exact morphological location of the metabolic event and secondly, ROIs can be made from CT or MRI images alone, which makes the placement of these more accurate and furthermore limits possible inter-observer errors. PET is indeed a fast-growing modality and both PET and cyclotron centers are opening and expanding in the western countries, thereby increasing accessibility for researchers in the field of orthopedics. We therefore find it very important that the limitations as well as the opportunities of this powerful tool are properly addressed and well understood to avoid misinterpretations and misunderstanding of this rather complex imaging modality.

Femoral head fractures: hemiarthroplasty or total hip arthroplasty?

Most femoral neck fractures are osteoporotic fractures in the elderly. The one-year mortality after neck fracture in this group is 24%. For hemiarthroplasty (HA) the bipolar heads have a risk reduction for reoperation due to acetabular erosion compared with monoblock heads. Surprisingly, the bipolar head had an increased reoperation risk for dislocation, infection and for periprosthetic fracture. Total hip arthroplasty (THA) after fracture has a four-fold raised risk for dislocation compared with THA after osteoarthritis. A larger head on the same neck (head to neck ratio) results in a theoretically larger range of movement and hence less risk for dislocation. The dual mobility bearing has, theoretically, the largest range of movement and good clinical results. Functional results are better for THA compared with HA. Arthroplasty for fracture has much better results compared with arthroplasty after a failed internal fixation; the risk for reoperation is more than doubled for the latter. A Swedish hip arthroplasty register study found a 20-fold higher risk for periprosthetic fracture when comparing uncemented HA with matt cemented HA. Also a polished cemented stem had 13½-fold higher risks compared with a matt. The mortality during the first day after surgery is higher for cemented compared with uncemented arthroplasties, but lower after one week, one month and one year. Analysing the time points together resulted in no difference. A matt cemented THA with a maximum head size, maybe dual mobility, has the best results, and is also for the low-demanding elderly.

Metabolic development of necrotic bone in the femoral head following resurfacing arthroplasty. A clinical [18F]fluoride-PET study in 11 asymptomatic hips.

Background and purpose One concern regarding resurfacing arthroplasty is the viability of the diminished femoral head and the postoperative risk of collapse, or a femoral neck fracture. 18F-fluoride positron emission tomography (F-PET) enables us to assess bone viability despite there being a covering metal component. By F-PET studies, we recently showed the absence of metabolism in the remaining part of femoral heads, 1–4 years after surgery in 11 of 46 consecutive cases. We now present the further development of bone metabolism in these 11 cases. Patients and methods 10 patients (11 chips) with previously shown loss of femoral head metabolism were evaluated by radiography and repeated F-PET scans, 3–6.5 years after surgery. The size of the area with low 18F-fluoride PET uptake in the femoral head was compared to that in earlier PET images. Results No patients had any clinical symptoms; nor was any necrotic bone area visible in plain radiographs. On F-PET scans, 2 patients showed a diminished area with low uptake, 4 were unchanged, and 5 had enlarged areas. Interpretation Bone metabolism surrounding a volume of bone with no metabolic activity changes dynamically even 5 years after surgery. The presence of bone with minor uptake of F-tracer, indicating low or no bone metabolism, with further progression in 5 of 11 cases leads us to conclude that resurfacing THA should be used restrictively.

Metal artifact reduction in 18F-fluoride-PET/CT data of prosthetic hip patients

Objective: Quantitative Iodine 124 PET/CT is a highly promising tool for pre therapeutic tumour dosimetry before Iodine 131 radionuclide therapy. However, imaging is complicated by prompt gamma coincidences (PGC) that add an undesired background activity to the images. An option for PGC correction is available as part of the modelled scatter correction on the GE Discovery 690 PET/CT system. We evaluated the effect of this correction on image quality. Materials and methods: I spatial resolution was measured using the NEMA NU 2 standard. Six spherical inserts with volumes between 0.52 and 26.52ml were filled with a homogeneous aqueous solution of I and placed inside the NEMA/IEC torso phantom. I activity was assessed in reconstructed PET/CT images of the phantom and the contrast recovery of each lesion was determined. The background of an 18cm high, 20cm diameter cylindrical phantom with three non radioactive 5cm diameter cylindrical rods (teflon, air and water) was filled with a homogeneous aqueous I solution. Four 4cm cylindrical ROIs were placed in the background of the PET/CT images and noise and uniformity were assessed. Similar regions were placed over each 5cm diameter rod and spill in of activity into the rods was determined. All reconstructions were done with and without PGC correction, and all assessments of I activities were compared to gamma ray spectroscopy measurements. Results: The spatial resolution was measured to 5.5mm axially and 4.7mm transversally. Visual image quality was greatly improved by PGC correction. Contrast recovery was improved by 4% for the smallest lesion and 6% for the largest lesion. The total activity assessed from the reconstructed images with and without PGC correction was 98% and 90%, respectively, of the expected value. Without PGC correction, the uniformity in the cylindrical phantom was clearly biased by an overcorrection of scatter in the central ROI and only 75% of the expected activity concentration was recovered. With PGC correction, uniformity and quantification was improved, though activity assessed in the central part of the phantom was 11% higher than expected. Excess activity was also measured in the “cold” cylindrical inserts, indicating that PGC and scatter was slightly under corrected. Conclusion: PGC correction improves image quality and quantification accuracy. High quantitative accuracy can be expected when the activity is limited to a few isolated lesions. In imaging situations with a high I background, quantification is challenged. A better modelling of PGC in the scatter correction could improve the situation.