Computer-assisted techniques are developed to optimise the positioning of acetabular cups in total hip replacement. However, ordinary guiding devices are still most commonly used. The aim of this study was to evaluate the accuracy when using a simple mechanical guiding device.

30 patients were operated by an experienced hip surgeon. A lateral position and a lateral approach were used. An un cemented press fit cup (Trilogy AB) was inserted using the guiding device for this type of pros-thesis, aiming 45 degrees abduction and 20 degrees ante-version. Radiological investigations were performed one week and three months postoperatively. Frontal views of the pelvis and of the operated hip were obtained. After scanning the contour of the opening of the acetabular prosthesis was identified and digitised using an edge detecting technique. The axes of the ellipsis of the acetabular opening served for calculation of the version of the cup. A lateral view clarified whether the cup was ante- or retroverted. The abduction related to the teardrop-line was measured on the scanned pelvic radiograph.

One week postoperatively mean abduction was 50 degrees (37–62), SD 5 degrees. Mean anteversion measured on the pelvic view was 9 degrees (2–23), SD 5 degrees, compared to 11 degrees (4–24), SD 5 degrees, on the frontal view of the hip joint 50 per cent of the cups showed a deviation of more than 10 degrees from the aimed anteversion.. At average the anteversion was 2 degrees lower when measured on the pelvic view compared to that measured on the AP-view of the hip. There were no significant differences between the measurements at one week and three months.

The anteversion of uncemented press fit acetabular components tends to be lower than intended when using a simple guiding device. The risk of an unacceptable abduction seems negligible. The inaccuracy in acetabular positioning may be due to inadequate positioning of the pelvis or inaccurate insertion technique.

In the past it has been widely accepted that bone remodelling of the proximal femur after cementless total hip replacement is a result of the altered mechanical environment. Usually, there is are distribution of the stresses in the bone, and subsequently bone mass, from the metaphysis to the proximal part of the diaphysis. The design rationale for some cementless stems is to transmit load to the proximal femur and thus to preserve the bone mineral content in this area. The aim of the present study was to investigate the relationship between postoperative strain shielding of the proximal femur and the bone remodelling after insertion of two different cementless femoral stems.

Experimental study: Twelve pairs of human cadaveric femurs were instrumented with strain gauge rosettes in Gruen zones2 to 7 and the cortical strains were measured during simulation of one leg stance before and after insertion of a custom stem (Unique, SCP) or an anatomic stem (ABG, Stryker-Howmedica).

Clinical study: In a prospective, randomized study including 80 patients, the same types of stems were inserted and the bone mineral density (BMD) was measured during the first two years postoperatively using DEXA. Then, the pattern of remodelling was compared with the gradient of strain shielding in each of the Gruen zones in the frontal plane.

In Gruen zone 7 the relative cortical strain shielding was45% in the femurs with a custom stem and 87% in the femurs with an anatomic stem. In zone 6 the corresponding figures were 2% and 38%, in zone 5 0% and15% and in zone 3 0% and 20%. The DEXA measurements showed a decrease in BMD in zone 7 of 22% and 23% for the two stems, respectively. In the other zones the bone loss was smaller and there was no difference between the stems.

In the proximal zones there was a highly significant difference in strain shielding between femurs receiving a customor an anatomic stem. However, there was no difference in the pattern of bone remodelling. The bone remodelling around these two stems does not seem to mirror the gradient of strain shielding.

A customised, uncemented femoral stem was introduced clinically in 1995 after several years of development and pre-clinical testing. All the patients operated in our hospital have entered a prospective clinical study. The aim of this study is to present the short-term clinical data. Furthermore, the measurement of implant migration and the periprosthetic bone remodelling at two years is also reported.

Materials and methods: The femoral stem is designed from preoperative CT-scans, machined in Ti-alloy and circumferentially coated with a 50μm hydroxyapatite (HA) layer in the proximal 50-70%. Fifty-one patients (median age 52 years) have been followed clinically for a minimum of 3 years using the Merle-d’Aubignè score. Migration of the femoral stem has been measured with radiostereometry (RSA), the precision of the measurements is better than 0.080 mm for translations and 0.30° for rotations. Periprosthetic bone remodelling is expressed as the change in bone mineral density (BMD) in seven zones (Gruen) relative to the postoperative values. RSA- and DEXA measurements have been performed postoperatively and then after 3, 6, 12 and 24 months.

Results: One stem had to be revised after 3 months due to a periprosthetic fracture. The clinical scores were as follows (preop/3 years): Pain 2.6/5.5, ROM 3.7/5.7, function 2.7/5.9, total score 9.1/17.1. Six patients complained of thigh pain during the first two years, however, this complication resolved spontaneously in five patients within the three years follow-up. The mean subsidence after two years was 0.055 mm (SE ±0.045 mm) and the mean axial rotation was 0.29° (SE ± 0.12°). The mean bone loss in zone 7 was 34%; in the other zones the bone loss was less than 14%. The mean overall bone loss was 8%.

Discussion: The short-term clinical experiences with this patient-specific, cementless femoral stem are encouraging. The stem seems to be very stable during the first two postoperative years indicating that biological fixation of the femoral stems has been achieved. The change in the BMD was less than 14% in all Gruen zones, except for the proximal medial area where the bone loss was 34%.

An argument against the use of canal-filling, customised femoral stems has been that such implants have a large cross-sectional area and therefore are stiffer than standard, uncemented implants, thus inducing more stress shielding and bone loss in the proximal femur. The purpose of this study was to evaluate the association between the volume of the femoral stem and the change in periprosthetic bone mineral density (BMD) measured with DEXA.

Material and methods. Forty-eight patients with a mean age of 46 years had a THR using a customised femoral stem (Unique, SCP, Norway). BMD was measured in the 7 Gruen zones postoperatively and after then 3, 6, 12 and 24 months. Based on the 3-D computer model of the implant the volume of the intrafemoral part of the stem was computed. The association between the relative change in BMD at the 2 years follow-up and the volume of the stem was assessed using correlation analyses.

Results. No statistically significant correlation between the volume of the intrafemoral part of the stem and change in BMD could be found for the most proximal zones (1, 6 and 7). Neither was there an association between the postoperative BMD value and the degree of stress shielding. However, a weak correlation between the volume of the stem and change in BMD was found in Gruen zones 2, 4, 5 and for the overall BMD in all zones.

Discussion. No consistent or strong association between the volume of the femoral stem and the periprosthetic stress shielding at 2 years postoperatively was found in this study. In particular, the bone remodeling in the most proximal part of the femur was not correlated to the size of the stem. Contrary to other studies, our findings imply that stem size or stiffness is not a major determinant for proximal femoral stress shielding following THR.

New prosthesis designs should be compared to a standard implant in randomized studies evaluated by radiostereometric analysis (RSA). The Unique customized prosthesis (UCP) is a newly developed concept for fitting uncemented prosthesis to the exact internal shape of the proximal femur [1]. We evaluated the new UCP design with the null hypothesis that this implant would be no more stable than a standard cemented implant.

Material and methods: 38 patients, mean age 51.6 years (31–65) were randomized to a UCP HA coated femoral stem or an Elite Plus (DePuy) cemented stem. All patients were implanted with a Duraloc (DePuy) uncemented cup except one patient with a protrusio who was primary impaction grafted with a cemented cup. Most patients received a Zirconium head, and all heads were 28 mm. The femoral stems were fitted with 3 tantalum balls and 4–10 tantalum balls were implanted in the femur during operation. RSA pictures were taken postoperatively, after 6 and 12 months.

Results: The Elite Plus stem rotated more into retroversion after 6 and 12 months (0.79° versus 0.31° after 12 months, P< 0.05). Nearly all of this rotation took place during the first 6 months. The Elite Plus stem migrated medially while the UCP migrated laterally (0.04 mm medially versus 0.03 mm laterally, P=0.06). The Elite Plus stem also migrated more distally than the UCP (0.17 mm versus −0.06 mm, P=0.055).

Discussion: Customized implants were more stable than the cemented Elite Plus prosthesis. Compared to other results with the Elite Plus rotations and migrations were small in this study [2]. Initially all patients had good clinical results, and only by long time follow up any clinical differences due to the small differences in stability as measured by RSA can be found.

Introduction: Several factors of which many still are unknown may influence the mechanical strength of the impacted morsellised bone applied in revision hip surgery. To avoid initial subsidence of the prosthesis can be crucial for the survival of the revised joint. Impacted grinded bone forms a porous structure where the void space mainly is filled by liquid consisting of water and fat.

Purpose of Study: To determine the influence of moisture on the stiffness strength of impacted morsellised bone.

Material and methods: Juxtaarticular bovine bone is morsellised and experimentally impacted by standardised methods. The stiffness of the bone was recorded during bone sample construction and loading the impacted bone sample, in a brief undrained and a longer drained phase. Water and fat content were measured in loose and compressed bone material and could be altered by reproducible methods. A trial on drainage during impaction was executed. All studies were compared with a base line study. This trial was carried out as similar as possible to the routine, clinical situation.

Results: Dried bone had very high axial stiffness properties. Lowering fat content resulted in bone samples, which had significantly higher stiffness during the undrained loading period but did not diverge during drained loading. Drainage during impaction had no effect on axial strength of the bone.

Conclusion: Low water content in morsellised bone has considerable positive effect on the axial strength of impacted bone. Fat lowering may have a positive effect during initial load phase but this advantage diminishes over time. In this experimental study draining through impaction did not improve axial strength. This is probably due to the transitory effect impaction has on porous pressure.

The aim of this paper is to present our 7 years experience with the use of a custom femoral stem with proximal HA-coating (Unique SCP). This prosthesis was developed to optimise the þxation and the strain distribution to the proximal femur and also the biomechanics of the hip in uncemented femoral stems. Methods: 226 hips have been operated. Mean age was 51.5 years (24–66). 44.7% of the hips were dysplastic. The prostheses were designed to optimise anteversion and leg length correction. All patients were followed with radiological and clinical examination. The maximum observation time is 7 years. Merle DñAubigne score was used. Results: Three patients sustained a peroperative þssure in the proximal femur (1.3%), the þssures were treated successfully with cerclage wires. One patient sustained a femoral fracture 3 months postoperatively. A dislocation of the joint occurred by severe injuries in three patients later than three months after the operation (1.3%). These joints have been stable after non-operative reduction. Thigh pain after two years was seen in only one patient. Average total score at 3 years (82 patients) was 5.68 (preop 3.12), at 5 years (28 patients) 5.65 (preop 3.12). The pain scores at the corresponding observations were 5.65 (preop 2.71) and 5.75 (preop 2.71). There have been no radiological signs of loosening or severe bone loss in observations up to 7 years postoperatively. Conclusions: The rate of peroperative þssures, postoperative dislocations and thigh pain is low in this type of prostheses compared to most type of uncemented standard stems. This is probably due to an optimised design. The mid-terms clinical results up to seven years postop are excellent.

Aims: The aim of this study was to measure implant migration and bone remodelling of the proximal femur two years after insertion of a customized or a standard femoral stem. Materials and methods: In a prospective, randomized study 26 hips (26 patients) have been examined postoperatively and after 3, 6, 12 and 24 months using radiostereometry (RSA) and DEXA. Thirteen hips received a customized femoral stem (Unique, SCP as) and 13 hips received a standard uncemented femoral stem (ABG¨, Stryker-Howmedica). An uncemented acetabular cup (Duraloc¨, DePuy) was used in all hips. The mean age of the patients was 55 (24–67) years. Results: The median displacement of the custom/ standard femoral stems was 0.04/0.01 mm along the - medial-lateral axis, 0.08/0.02 mm along the proximal-distal axis and 0.03/0.08 mm along the anterior-posterior axis, respectively. Statistically, there was no difference between the two groups. One custom stem subsided 5.2 mm at one year, but showed no further migration at two years. The mean decrease in bone mineral density (BMD) in all Gruen zones was 6% in the Custom-group and 7% in the ABG-group. The most pronounced bone loss was seen in Zone 7 and was 21% and 25% for the two groups, respectively. Discussion: We found no statistically signiþcant difference in short-term stem migration comparing a customized and a standard, uncemented femoral stem. Furthermore, the changes in bone mineral density were almost equal in femurs with either type of prosthesis.

Aims: An argument against the use of canal-þlling, customized femoral stems has been that such implants have a large cross-sectional area and therefore are stiffer than standard, uncemented implants and thus induce more stress shielding and bone loss in the proximal femur. The purpose of this study was to evaluate the association between the volume of an uncemented, customized femoral stem and the postoperative changes in peri-prosthetic bone mineral density (BMD) measured with DEXA.

Material and methods: Forty-eight patients with a mean age of 46 years had a THR using a customized femoral stem (Unique, SCP, Norway). BMD was measured in the 7 Gruen zones postoperatively and after then 3, 6, 12 and 24 months. Based on the 3-D computer model of the implant the volume of the intrafemoral part of the stem was computed. The association between the relative change in BMD at the 2 years follow-up and the volume of the stem was assessed using Pearsonñs correlation test.

Results: A statistically signiþcant correlation between the volume of the intrafemoral part of the stem and change in BMD was found in Gruen zones 2, 4, 5 and for the overall BMD in all zones. No such correlation could be found for the most proximal zones (1, 6 and 7).

Conclusions: No consistent or strong association between the volume of the femoral stem and the periprosthetic stress shielding at 2 years postoperatively was found in this study. In particular, the bone remodelling in the most proximal part of the femur was not correlated to the size of the stem. Contrary to other studies, our þndings imply that stem size or stiffness is not a major determinant for proximal femoral stress shielding following total hip replacement.

Aims: The conventional arm prosthesis used by transhumeral (TH) amputees has shoulder harness with straps around the contralateral shoulder. Thus, the arm prosthesis has a limited range of motion and stability. Patient complaints of pain from the neck and the contralateral shoulder are common. We surgically modiþed the TH amputation stump by use of a cemented T-Endo-Prostesis (TEP). We studied the functional results after employing a new TH arm prosthesis without shoulder harness and straps around the contralateral shoulder. Methods: 3 TH amputees were operated with implantation of a TEP (Fig 1). Standard high pressure cementing technique was used. Results: Patient I (12 mnd postop): The patient used a new TH arm prosthesis (Fig 2 and 3). The prosthesisñ main grip was around the new ÒcondylesÒ. No shoulder harness or straps were needed. The shoulder abduction with prosthesis had increased from 90û to 150û. The maximum rotational force with 90û elbow ßexion had increased from 9 to 30 N (in) and 8 to 20 N (out). Previous pain from the neck and the contralateral shoulder had disappeared. Patient II (6 mnd postop): The shoulder abduction with prosthesis had increased from 100û to 150û. Pain from the neck and the contralateral shoulder had disappeared. Patient III (3 mnd postop): The patient has þnished antioedema regime and has started adaptation of the new prosthesis. Conclusion: Preliminary results from this experimental clinical study indicate that when the TH amputation stump is surgical modiþed with a TEP, a new type of TH arm prosthesis gives a better functional result.

CT-based, customised femoral stem enables optimal reconstruction of hip mechanics and leg length. However, traditional planning and execution of cup insertion may jeopardise these biomechanical parameters. The aim of this study was to examine the agreement of the preoperative planning of cup position and the final position of the cup.

Thirty total hip replacements with an uncemented acetabular cup (Duraloc, DePuy) or a cemented cup (Elite-Plus, DePuy) were included. A customised femoral stem was used in all hips. On the preoperative X-rays the planned position and orientation of the cup had been marked prior to the surgery. The pre- and postoperative X-ray images were then digitised and scaled. The planned and final positions of the cup centre in the frontal plane was then measured relative to a horizontal line defined by the tear-drops and to a vertical line through the centre of the tear-drop on the operated side. In addition the concurrence between the planned and final cup size was examined.

In the horizontal direction the cups were positioned 1.4 (7.6) mm (median, ±2SD) more medial than planned on the preoperative X-rays. In the vertical direction the corresponding figures were 1.2 (6.6) mm (median, ±2SD) and the cups were usually placed more cranially than was planned. The maximum discrepancy between the planned and final position was 10,6 mm in the horizontal direction (medial) and 7.1 mm in the vertical direction (cranial). In 63% of the hips there was agreement between the size of the cup planned preoperatively and the cup that was finally inserted. In 25% of the hips the final cup was larger and in 12% the final cup was smaller.

In most cases the acetabular cups were inserted within a few millimetres of the planned position. The combination of a standard uncemented or cemented cup with a custom femoral stem enables the surgeon to restore hip mechanics and leg length.