Risk for hip fracture before and after total knee replacement in Sweden.

We studied the risk for hip fracture before and after total knee replacement (TKR) in the entire population in Sweden. Women and men had a low risk for hip fracture before TKR but an increased risk the first year after TKR. PURPOSE: It is known that osteoarthritis is associated with high bone mass. We therefore studied the risk of hip fracture before and after total knee replacement (TKR), risk of different hip fracture types, and risk subdivided in genders and age groups. METHODS: We followed the total Swedish population born between 1902 and 1952 (n = 4,258,934) during the period 1987-2002 and identified all patients with TKR due to primary OA (n = 39,291), and all patients with hip fracture (n = 195,860) in the Swedish National Inpatient Register. The risk time analyses were based on Poisson regression models. RESULTS: The hazard ratio (HR) for hip fracture the last year before TKR was 0.86 (95% CI 0.74 to 1.00) and the first year after 1.26 (95% CI 1.11 to 1.42) compared to individuals without TKR. The HR for femoral neck fracture 0-10 years after TKR was 0.95 (95% CI 0.89 to 1.01) and for trochanteric fracture was 1.13 (95% CI 1.06 to 1.21). The HR for hip fracture in the age group 50-74 was 1.28 (95% CI 1.14 to 1.43) and in the age group 75-90 years was 0.99 (95% CI 0.94 to 1.04) 0-10 years after TKR, compared to individuals without TKR. CONCLUSION: Individuals had a low risk for hip fracture before TKR but an increased risk the first year after TKR. The risk in individuals below age 75 years and for trochanteric fractures was increased after TKR. Possible explanations include changed knee kinematics after a TKR, physical activity level, fall risk, and other unknown factors.

Low risk for hip fracture and high risk for hip arthroplasty due to osteoarthritis among Swedish farmers.

We aimed to study the risk of hip fracture and risk of hip arthroplasty among farmers in Sweden. Our results indicate that farming, representing an occupation with high physical activity, in men is associated with a lower risk of hip fracture but an increased risk of hip arthroplasty. INTRODUCTION: The risks of hip fracture and hip arthroplasty are influenced by factors including socioeconomic status, education, urbanization, latitude of residence, and physical activity. Farming is an occupation encompassing rural living and high level of physical activity. Therefore, we aimed to study the risk of hip fracture and risk of hip arthroplasty among farmers in Sweden. METHODS: We studied the risk of hip fracture, and hip arthroplasty due to primary osteoarthritis, in all men and women aged 35 years or more in Sweden between 1987 and 2002. Documented occupations were available in 3.5 million individuals, of whom 97,136 were farmers. The effects of age, sex, income, education, location of residence, and occupation on risk of hip fracture or hip arthroplasty were examined using a modification of Poisson regression. RESULTS: A total of 4027 farmers and 93,109 individuals with other occupations sustained a hip fracture, while 5349 farmers and 63,473 others underwent a hip arthroplasty. Risk of hip fracture was higher with greater age, lower income, lower education, higher latitude, and urban area for all men and women. Compared to all other occupations, male farmers had a 20% lower age-adjusted risk of hip fracture (hazard ratio (HR) 0.80, 95%CI 0.77-0.84), an effect that was not seen in female farmers (HR 0.96, 95% CI 0.91-1.01). Both male and female farmers had a higher age-adjusted risk for hip arthroplasty. CONCLUSIONS: Our results indicate that farming, representing an occupation with high physical activity, in men is associated with a lower risk of hip fracture but an increased risk of hip arthroplasty.

Does hydroxyapatite coating of uncemented cups improve long-term survival? An analysis of 28,605 primary total hip arthroplasty procedures from the Nordic Arthroplasty Register Association (NARA).

OBJECTIVE: It is unclear whether hydroxyapatite (HA) coating of uncemented cups used in primary total hip arthroplasty (THA) improves bone ingrowth and reduces the risk of aseptic loosening. We therefore investigated survival of different uncemented cups that were available with or without HA coating. METHOD: We investigated three different cup types used with or without HA coating registered in the Nordic Arthroplasty Register Association (NARA) database that were inserted due to osteoarthritis (n = 28,605). Cumulative survival rates and adjusted hazard ratios (HRs) for the risk of revision were calculated. RESULTS: Unadjusted 13-year survival for cup revision due to aseptic loosening was 97.9% (CI: 96.5-99.4) for uncoated and 97.8% (CI: 96.3-99.4) for HA-coated cups. Adjusted HRs were 0.66 (CI 0.42-1.04) for the presence of HA coating during the first 10 years and 0.87 (CI 0.14-5.38) from year 10-13, compared with uncoated cups. When considering the endpoint cup revision for any reason, unadjusted 13-year survival was similar for uncoated (92.5% [CI: 90.1-94.9]) and HA-coated (94.7% [CI: 93.2-96.3]) cups. The risk of revision of any component due to infection was higher in THA with HA-coated cups than in THA with uncoated cups (adjusted HR 1.4 [CI 1.1-1.9]). CONCLUSIONS: HA-coated cups have a similar risk of aseptic loosening as uncoated cups, thus the use of HA coating seems to not confer any added value in terms of implant stability. The risk of infection seemed higher in THA with use of HA-coated cups, an observation that must be investigated further.

Association between hospital procedure volume and risk of revision after total hip arthroplasty: a population-based study within the Nordic Arthroplasty Register Association database.

OBJECTIVE: Outcome after total hip arthroplasty (THA) depends on several factors related to the patient, the surgeon and the implant. It has been suggested that the annual number of procedures per hospital affects the prognosis. We aimed to examine if hospital procedure volume was associated with the risk of revision after primary THA in the Nordic countries from 1995 to 2011. DESIGN: The Nordic Arthroplasty Register Association database provided information about primary THA, revision and annual hospital volume. Hospitals were divided into five volume groups (1-50, 51-100, 101-200, 201-300, >300). The outcome of interest was risk of revision 1, 2, 5, 10 and 15 years after primary THA. Multivariable regression was used to assess the relative risk (RR) of revision. RESULTS: 417,687 THAs were included. For the 263,176 cemented THAs no differences were seen 1 year after primary procedure. At 2, 5, 10 and 15 years the four largest hospital volume groups had a reduced risk of revision compared to group 1-50. After 10 years RR was for volume group 51-100 0.79 (CI 0.65-0.95), group 101-200 0.76 (CI 0.61-0.95), group 201-300 0.74 (CI 0.57-0.96) and group >300 0.57 (CI 0.46-0.71). For the uncemented THAs an association between hospital volume and risk of revision were only present for hospitals producing 201-300 THAs per year, beginning at years 2 through 5 and in all subsequent time intervals to 15 years. CONCLUSION: Hospital procedure volume was associated with a long term risk of revision after primary cemented THA. Hospitals operating 50 procedures or less per year had an increased risk of revision after 2, 5, 10 and 15 years follow up.

Association between fixation technique and revision risk in total hip arthroplasty patients younger than 55 years of age. Results from the Nordic Arthroplasty Register Association.

OBJECTIVES: To evaluate implant survival following primary total hip replacement (THR) in younger patients. To describe the diversity in use of cup-stem implant combinations. DESIGN: 29,558 primary THRs osteoarthritis (OA) patients younger than 55 years of age performed from 1995 through 2011 were identified using the Nordic Arthroplasty Registry Association database. We estimated adjusted relative risk (aRR) of revision with 95% confidence interval (CI) using Cox regression. RESULTS: In general, no difference was observed between uncemented and cemented implants in terms of risk of any revision. Hybrid implants were associated with higher risk of any revision (aRR = 1.3, CI: 1.1-1.5). Uncemented implants led to a reduced risk of revision due to aseptic loosening (aRR = 0.5, CI: 0.5-0.6), whereas the risk was similar for hybrid and cemented implants. Compared with cemented implants, both uncemented and hybrid implants led to elevated risk of revision due to other causes, as well as elevated risk of revision due to any reason within 2 years. 183 different uncemented cup-stem implant combinations were registered in Denmark, of these, 172 were used in less than 100 operations which is similar to Norway, Sweden and Finland. CONCLUSIONS: Uncemented implants perform better in relation to long-term risk of aseptic loosening, whereas both uncemented and hybrid rather than cemented implants in patients younger than 55 years had more short-term revisions because problems due to dislocation, periprosthetic fracture and infection has not yet been completely solved. The vast majority of cup-stem combinations were used in very few operations.

Evaluation of in vivo wear of vitamin E-diffused highly crosslinked polyethylene at five years: a multicentre radiostereometric analysis study.

AIMS: The primary aim of this study was to compare the wear properties of vitamin E-diffused, highly crosslinked polyethylene (VEPE) and one formulation of moderately crosslinked and mechanically annealed ultra-high molecular weight polyethylene (ModXLPE) in patients five years after primary total hip arthroplasty (THA). The secondary aim was to assess the clinical results of patients treated with VEPE by evaluating patient-reported outcome measures (PROMs), radiological evidence of fixation, and the incidence of mechanical failure. PATIENTS AND METHODS: A total of 208 patients (221 THAs) from four international centres were recruited into a prospective study involving radiostereometric analysis (RSA) and the assessment of clinical outcomes. A total of 193 hips (87%) were reviewed at the five-year follow-up. Of these, 136 (70%) received VEPE (vs ModXLPE) liners and 68 (35%) received ceramic (vs metal) femoral heads. PROMs and radiographs were collected preoperatively and at one, two, and five years postoperatively. In addition, RSA images were collected to measure PE wear postoperatively and at one, two, and five years after surgery. RESULTS: We observed similar bedding in one year postoperatively and wear two years postoperatively between the two types of liner. However, there was significantly more penetration of the femoral head in the ModXLPE cohort compared with the VEPE cohort five years postoperatively (p < 0.001). The only variables independently predictive of increased wear were ModXLPE (vs VEPE) liner type (ß = 0.22, p = 0.010) and metal (vs ceramic) femoral head (ß = 0.21, p = 0.013). There was no association between increased wear and the development of radiolucency (p = 0.866) or PROMs. No patient had evidence of osteolysis. CONCLUSION: Five years postoperatively, patients with VEPE (vs ModXLPE) and ceramic (vs metal) femoral heads had decreased wear. The rates of wear for both liners were very low and have not led to any osteolysis or implant failure due to aseptic loosening.

Influence of joint area design on tibial component migration: comparison among a fixed symmetrical, asymmetrical, and moveable bearing.

Fifty-four knees (50 patients) were allocated to three different tibial polyethylene inserts (standard/rotating platform/FS1000) in the Freeman-Samuelson (Finsbury Orthopaedics Ltd, Surrey, United Kingdom) total knee arthroplasty. The FS1000 design has a spherical medial and a roller-in-trough configuration laterally. Radiostereometric examinations were done postoperatively and after 3, 12, and 24 months. The median migration of the metal-backing and the Hospital for Special Surgery scores did not differ between the three groups. At 2 years, the median external/ internal displacements for the rotating platforms were 2.8 degrees and 0.2 degrees, respectively (rotating platforms versus standard inserts/rotating platforms versus FS1000: P < .0005). Longitudinal rotations above the detection limits for radiostereometry were observed in some of the fixed implants (standard and FS1000 designs), indicating conditions for backside wear.

Promising early results for trabecular metal acetabular components used at revision total hip arthroplasty: 42 acetabular revisions followed with radiostereometry in a prospective randomised trial.

AIMS: The aim of this study was to compare the incidence of aseptic loosening after the use of a cemented acetabular component and a Trabecular Metal (TM) acetabular component (Zimmer Inc., Warsaw, Indiana) at acetabular revision with bone impaction grafting. PATIENTS AND METHODS: A total of 42 patients were included in the study. Patients were randomised to receive an all- polyethylene cemented acetabular component (n = 19) or a TM component (n = 23). Radiostereometric analysis and conventional radiographic examinations were performed regularly up to two years post-operatively or until further revision. RESULTS: The proximal migration was significantly higher in the cemented group. At two years, the median proximal migration was 1.45 mm and 0.25 mm in the cemented and TM groups, respectively (p = 0.02). One cemented component was revised due to dislocation. There were no revisions in the TM group. CONCLUSION: Lower proximal migration in the TM group suggests that this design might be associated with a lower risk of aseptic loosening in the long term compared with an all polyethylene cemented component. Longer follow-up is required to confirm the clinical advantages of using this component at acetabular revision. Cite this article: Bone Joint J 2017;99-B:880-6.

Cement-in-cement revision of the femoral stem: analysis of 1179 first-time revisions in the Swedish Hip Arthroplasty Register.

AIMS: Compared with primary total hip arthroplasty (THA), revision surgery can be challenging. The cement-in-cement femoral revision technique involves removing a femoral component from a well-fixed femoral cement mantle and cementing a new stem into the original mantle. This technique is widely used and when carried out for the correct indications, is fast, relatively inexpensive and carries a reduced short-term risk for the patient compared with the alternative of removing well-fixed cement. We report the outcomes of this procedure when two commonly used femoral stems are used. PATIENTS AND METHODS: We identified 1179 cement-in-cement stem revisions involving an Exeter or a Lubinus stem reported to the Swedish Hip Arthroplasty Register (SHAR) between January 1999 and December 2015. Kaplan-Meier survival analysis was performed. RESULTS: Survivorship is reported up to six years and was better in the Exeter group (91% standard deviation (sd) 2.8% versus 85% sd 5.0%) (p = 0.02). There was, however, no significant difference in the survival of the stem and risk of re-revision for any reason (p = 0.58) and for aseptic loosening (p = 0.97), between revisions in which the Exeter stem (94% sd 2.2%; 98% sd 1.6%) was used compared with those in which the Lubinus stem (95% sd 3.2%; 98% sd 2.2%) was used. The database did not allow identification of whether a further revision was indicated for loosening of the acetabular or femoral component or both. CONCLUSION: The cement-in-cement technique for revision of the femoral component gave promising results using both designs of stem, six years post-operatively. Cite this article: Bone Joint J 2017;99-B(4 Supple B):27-32.

Risk of early mortality after cemented compared with cementless total hip arthroplasty: a nationwide matched cohort study.

AIMS: It has been suggested that cemented fixation of total hip arthroplasty (THA) is associated with an increased peri-operative mortality compared with cementless THA. Our aim was to investigate this through a nationwide matched cohort study adjusting for age, comorbidity, and socioeconomic background. PATIENTS AND METHODS: A total of 178 784 patients with osteoarthritis who underwent either cemented or cementless THA from the Swedish Hip Arthroplasty Register were matched with 862 294 controls from the general population. Information about the causes of death, comorbidities, and socioeconomic background was obtained. Mortality within the first 90 days after the operation was the primary outcome measure. RESULTS: Patients who underwent cemented THA had an increased risk of death during the first 14 days compared with the controls (hazard ratio (HR) 1.3, confidence interval (CI) 1.11 to 1.44), corresponding to an absolute increase in risk of five deaths per 10 000 observations. No such early increase of risk was seen in those who underwent cementless THA. Between days 15 and 29 the risk of mortality was decreased for those with cemented THA (HR 0.7, CI 0.62 to 0.87). Between days 30 and 90 all patients undergoing THA, irrespective of the mode of fixation, had a lower risk of death than controls. Patients selected for cementless fixation were younger, healthier and had a higher level of education and income than those selected for cemented THA. A supplementary analysis of 16 556 hybrid THAs indicated that cementation of the femoral component was associated with a slight increase in mortality up to 15 days, whereas no such increase in mortality was seen in those with a cemented acetabular component combined with a cementless femoral component. CONCLUSION: This nationwide matched cohort study indicates that patients receiving cemented THA have a minimally increased relative risk of early mortality that is reversed from day 15 and thereafter. The absolute increase in risk is very small. Our findings lend support to the idea that cementation of the femoral component is more dangerous than cementation of the acetabular component. Cite this article: Bone Joint J 2017;99-B:37-43.

Imaging knee position using MRI, RSA/CT and 3D digitisation.

The purpose of this study was to compare 3 methods of imaging knee position. Three fresh cadaver knees were imaged at 6 flexion angles between 0 degrees and 120 degrees by MRI, a combination of RSA and CT and 3D digitisation (in two knees). Virtual models of all 42 positions were created using suitable computer software. Each virtual model was aligned to a newly defined anatomically based Cartesian coordinate system. The angular rotations around the 3 coordinate system axes were calculated directly from the aligned virtual models using rigid body kinematics and found to be equally accurate for the 3 methods. The 3 rotations in each knee could be depicted using anatomy-based diagrams for all 3 methods. We conclude that the 3 methods of data acquisition are equally and adequately accurate in vitro. MRI may be the most useful in vivo.

Three-dimensional knee joint movements during a step-up: evaluation after anterior cruciate ligament rupture.

The three-dimensional motions of the knee were analysed during closed kinetic chain knee extension in 13 patients with unilateral chronic injury of the anterior cruciate ligament. The patients ascended a platform, and serial stereophotogrammetric roentgenograms were exposed from about 100 degrees of flexion to full extension. From a position of about 100 degrees of knee flexion and 20 degrees of internal rotation, the tibia rotated externally during the extension. Almost no tibial adduction or abduction was observed. The tibial intercondylar eminence translated laterally, distally, and anteriorly relative to the femur. In knees with absence of the anterior cruciate ligament, the intercondylar eminence had a more posterior position compared with the contralateral normal knees. The proximal tibia was used as a fixed reference segment to evaluate the anteroposterior translations of a central point in the femoral condyles. The femoral point was more anteriorly displaced in the injured than in the contralateral knees. This difference might reflect increased activity of the hamstrings in the injured knees, because it was most pronounced at 80 degrees of flexion and decreased with increasing extension. In the sagittal plane, the mean helical axis was positioned close to the femoral insertion of the ligament at 80 degrees of flexion and was displaced distally and anteriorly during extension. In the frontal plane, the axis had a transverse direction at 80 degrees of flexion. At close to full extension, the axis was positioned distally in the lateral condyle and proximally in the medial condyle. In the horizontal plane, the helical axes ran slightly more anteriorly in the medial than in the lateral femoral condyle but changed inclination at close to full extension and became almost parallel to the transverse axis.

Three-dimensional knee kinematics and stability in patients with a posterior cruciate ligament tear.

The three-dimensional kinematics of the knee were studied from 5 months to 15 years after unilateral posterior cruciate ligament tears occurred in eight patients. All but two patients had signs of additional ligament injuries. Repeated radiostereometric examinations were conducted when the patients ascended a platform (step-up test) and during an instrumented anterior-posterior drawer test with the knee at 30 degrees of flexion. No changes in knee kinematics were observed during the step-up test, whereas increased anterior-posterior laxity (3.8-11.3 mm) was recorded for all patients. Four of the patients had an increased side-to-side difference (more than 2 mm) in anterior as well as posterior laxity. A rupture of the posterior cruciate ligament can be diagnosed at 30 degrees of knee flexion, but an increase in anterior laxity can erroneously be interpreted as an injury of the anterior cruciate ligament. The unaffected kinematics of the knee suggest that factors such as joint load and congruity and muscle activity can compensate for the absent posterior cruciate ligament during static examinations.

Migration of porous coated acetabular prostheses fixed with screws: roentgen stereophotogrammetric analysis.

The fixation of screw-fixed, porous-coated acetabular prostheses was studied during 2 years in 21 patients (22 hips) using repeated roentgen stereophotogrammetric measurements. Migration of the center of the prostheses and/or tilting were recorded in five cups after 6 months. Two years after operation, migration was recorded in three, migration and rotation in six, and only rotation in three cups. Bone quality, prosthetic position, and immediate postoperative radiographic appearance of the prosthesis-bone interface did not correlate with the occurrence of migration. Narrow radiolucent zones had developed in half of the patients after 2 years and was associated with proximal migration. Micromovements of screw-fixed, press-fit cups seem to start later compared with previously presented results from cemented acetabular prosthesis.

Inducible displacements of cemented tibial components during weight-bearing and knee extension observations during dynamic radiostereometry related to joint positions and 2 years history of migration in 16 TKR.

The inducible displacements of the tibial component caused by active extension were studied in 16 knees 1 yr after an AMK total knee arthroplasty with either flat. concave or posterior-stabilised (PS) designs of the joint area. Continuous change of the position of the tibial component occurred with proceeding extension. Rocking, subsidence and lift-off at different localisations were observed. In 3 of 4 knees with flat inserts the tibial component tilted anteriorly from 45 degrees to 35 degrees of flexion. A similar anterior tilt was seen in 2 of 6 with concave inserts and 5 of 6 with the PS design, but the tilting started later, when the knee had 5-20 degrees more extension. From 45-15 degrees of flexion most components tilted into valgus. Three knees (1 concave with, 1 concave without PCL and 1 PS) showed a sudden tilt into varus direction followed by a rocking motion in the opposite direction. The other types of displacements studied showed a more uniform pattern. The inducible maximum translation (MTPM) at 20 degrees of extension tended to be associated with increased migration between 0 and 2 yr when measured with the same parameter (Spearman's rho = 0.54, P = 0.03). Increased medial displacement of the center of the proximal tibia at 25 degrees was associated with increased anterior tilt. This type of motion was most commonly seen with the concave design. Our observations demonstrate that the forces acting on the tibial component vary during active extension, which results in rocking movements. This will influence the migration and the patttern of wear, factors of importance for the clinical longevity of a total knee replacement (TKR).

Femoral component migration in total knee arthroplasty: randomized study comparing cemented and uncemented fixation of the Miller-Galante I design.

The Miller-Galante I knee replacement was inserted in 25 women and three men (33 knees) with osteoarthrosis. All patients received a TiAlV femoral component with a commercially pure titanium fiber-mesh undersurface. Cemented or cementless fixation was used based on a randomization protocol. Micromotions of the femoral components were recorded with roentgen stereophotogrammetric analysis during the first 2 postoperative years. The magnitude of migration did not differ between cemented and uncemented fixation. The number of nonmigrating prostheses decreased from 21 (12 cemented and nine uncemented) at 3 months to six (three cemented and three uncemented) at 24 months. In both groups, the magnitude of prosthetic tilting about the longitudinal axis (internal-external rotation) was as large as that about the transverse axis (flexion-extension). Rotation into extension was as common as rotation into flexion. The largest translations were recorded at either of the posterior condyles. In 10 uncemented components, radiolucent lines were seen at the distal interface postoperatively. Proximal migration of the femoral component was recorded in these knees, and the width of the lines decreased or the lines disappeared totally at 24 months. After 2 years, lines were noted around four cemented and four uncemented replacements, mainly anteriorly or distally. All of these prostheses migrated. One prosthesis, revised because of malalignment, displayed pronounced migration after an initial period of stability. Bone ingrowth was observed anteriorly and anterodistally despite the presence of motions of 1 mm or more.

Migration of press-fit cups fixed with poly-L-lactic acid or titanium screws: a randomized study using radiostereometry.

The early migration of porous acetabular cups was studied in 43 patients with osteoarthrosis. The patients were randomly allocated to additional fixation of the cup with either biodegradable poly-L-lactic acid screws or titanium screws. Radiostereometric evaluation was done during the first 2 years after the procedure in 43 hips (23 with poly-L-lactic acid screws and 20 with titanium screws). At the 2-year follow-up, cups fixed with poly-L-lactic acid screws had migrated significantly more in the proximal-distal (p < 0.05) and medial-lateral (p < 0.05) directions. Cups with titanium screws displayed more pronounced rotations around the longitudinal axis (p < 0.05). Postoperatively, on the lateral view, there was an increased occurrence of radiolucencies at the dome of the cups fixed with poly-L-lactic acid screws (p < 0.05). The clinical result did not differ between the two groups. Inferior implant-bone contact in the poly-L-lactic acid group, local changes of the bone quality, and diminishing support of the poly-L-lactic acid screws caused by their degradation with time could be reasons for the different pattern of migration observed.

Deformation across the zone of callotasis during loading. radiostereometric analysis in a patient with achondroplasia.

The present study demonstrates that high-resolution radiostereometric analysis (RSA) can be used to assess global longitudinal compressive deformation across the callotasis zone during loading. In an achondroplastic patient operated with bifocal lengthening of the tibia by use of the Ilizarov external fixator, the axial compressive intersegmental strain in the proximal lengthening zone under a load of 71% of body weight was 7.7 mm. The proximal lengthening zone was 51.0 mm, and accordingly the overall linear strain across the callotasis was 15.1%. This large strain value found in distraction osteogenesis 6 weeks after end of distraction is not consistent with classical theory of the magnitude of micromotion needed for adequate stimulation of bone formation in fracture healing. The increased axial displacement did not stimulate bone healing and delayed union was observed. This one single observation does not allow for any conclusions to be drawn about the relationship of strain to fracture healing, but further and refined use of the RSA method will certainly improve our understanding of the role of axial strains in distraction osteogenesis.

In vivo kinematics of total knee arthroplasty: flat compared with concave tibial joint surface.

This study evaluated the influence of the geometric configuration of the tibial joint area on the kinematics of the knee. Twenty-two patients with noninflammatory arthritis and minor preoperative deformity were studied. They each received an AMK total knee replacement with retention of the posterior cruciate ligament. Eleven patients without any knee abnormalities were used as controls. The patients were stratified to either the flat (terminology of the manufacturer: standard) or concave (terminology of the manufacturer: constrained) polyethylene insert (n = 11 in each group). Knee kinematics were assessed 1 year after the operation by having the patient ascend a platform corresponding to an extension of the knee from 50 to 70 degrees of flexion. During this motion, two film-exchangers simultaneously exposed six to 13 pairs of serial stereoradiographs. The concave geometric configuration of the tibial insert resulted paradoxically in increased anterior-posterior translations compared with the flat insert but no significant change of rotations and translations in the other directions. Compared with normal knees, the most obvious abnormality was increased anterior-posterior translations (p < 0.004). At 50 degrees of flexion, the implants with the flat tibial polyethylene insert had displaced 2 times and the concave ones had displaced 2.5 times more posteriorly than the normal knees (p < or = 0.001). Less internal tibial rotation was also recorded in the flexed positions for both types of inserts compared with the normal knees (p < 0.02). Four knees in four patients, who reported symptoms of instability and abnormal knee function, showed significantly increased proximal displacement of the center of the tibial plateau in the flexed position. The findings suggest that current prosthetic designs and surgical technique do not restore normal knee kinematics and indicate that design improvements should rely on in vivo kinematic studies.

Micromotion of femoral stems in total hip arthroplasty. A randomized study of cemented, hydroxyapatite-coated, and porous-coated stems with roentgen stereophotogrammetric analysis.

The fixation of the femoral stem in a total hip arthroplasty was studied in sixty patients (sixty-four hips) with use of roentgen stereophotogrammetric analysis. The hips were randomly stratified on the basis of the age, sex, and weight of the patient; the roentgenographic quality of the bone; and the reason for the operation (primary or secondary osteoarthrosis). The hips were then randomly assigned to one of three types of fixation of the femoral component: insertion with cement, hydroxyapatite coating, or porous coating. Examinations with roentgen stereophotogrammetry were done as long as two years after the operation. Micromotion of the prostheses was evaluated in terms of subsidence or proximal migration and rotations and translations of the proximal-lateral (shoulder) and distal (tip) parts of the prostheses. The clinical results at the two-year follow-up evaluation did not differ significantly between the groups with regard to the Harris hip score or the pain score (p > 0.05 for both; Wilcoxon rank-sum test). There were small or no differences in rotations and micromotions of the shoulder and the tip of the prostheses. Increased subsidence of 0.1 to 0.2 millimeter was recorded for the cemented and the porous-coated prostheses (p = 0.002 and p = 0.02, respectively; Wilcoxon rank-sum test). Thus, proximal hydroxyapatite coating seems to enhance the early fixation of the stem. Conventional roentgenography revealed an increased number of radiodense lines surrounding the porous-coated prostheses. Distal hypertrophy of the femoral cortex was found mainly around the hydroxyapatite-coated prostheses. Proximal resorption of bone and heterotopic ossification occurred to approximately the same extent regardless of the type of fixation used.