[Imaging in joint-preserving hip surgery]. / Bildgebung in der gelenkerhaltenden Hüftchirurgie.

Instability and impingement of the hip are the main pathomechanisms that can lead to chondrolabral damage, painful restriction of motion and early hip osteoarthritis due to increased mechanical stress, even in young patients. The goals of joint-preserving hip surgery are correction of the osseous deformities and chondrolabral damage as well as restoration of joint function. A prerequisite for successful surgery is the identification of the underlying hip pathologies, which can also occur in combination. Dedicated radiographic and magnetic resonance (MR) imaging of osseous morphology and the internal degenerative joint lesions play an essential role for the treatment indications and surgical treatment planning. This article provides a concise overview of the prevalence, pathomechanisms and indicated treatment of hip deformities as well as detailed recommendations on the specific radiological clarification.

Femoral cartilage damage occurs at the zone of femoral head necrosis and can be accurately detected on traction MR arthrography of the hip in patients undergoing joint preserving hip surgery.

The primary purpose was to answer the following question: What is the location and pattern of necrosis and associated chondrolabral lesions and can they be accurately detected on traction MR arthrography compared with intra-operative findings in patients undergoing hip preservation surgery for femoral head necrosis (FHN)? Retrospective, diagnostic case series on 23 patients (23 hips; mean age 29 ± 6 years) with diagnosis of FHN undergoing open/arthroscopic joint preserving surgery for FHN and pre-operative traction MR arthrography of the hip. A MR-compatible device for weight-adapted application of leg traction (15-23 kg) was used and coronal, sagittal and radial images were acquired. Location and pattern of necrosis and chondrolabral lesions was assessed by two readers and compared with intra-operative findings to calculate diagnostic accuracy of traction MR arthrography. On MRI all 23 (100%) hips showed central FHN, most frequently antero-superiorly (22/23, 96%) where a high prevalence of femoral cartilage damage was detected (18/23, 78%), with delamination being the most common (16/23, 70%) damage pattern. Intra-operative inspection showed central femoral head cartilage damage most frequently located antero-superiorly (18/23, 78%) with femoral cartilage delamination being most common (14/23, 61%). Traction MR arthrography enabled detection of femoral cartilage damage with a sensitivity/specificity of 95%/75% for reader 1 and 89%/75% for reader 2. To conclude, femoral cartilage damage occurs at the zone of necrosis and can be accurately detected using traction MR arthrography of the hip which may be helpful for surgical decision making in young patients with FHN.

Proof of concept: hip joint damage occurs at the zone of femoroacetabular impingement (FAI) in an experimental FAI sheep model.

OBJECTIVE: In ovine hips chondrolabral damage as seen in cam-type femoroacetabular impingement (FAI) can be induced via an intertrochanteric varus osteotomy. However, it is yet to proven whether the observed cartilage damage is caused by a dynamic cam type impingement. Thus we asked, (1) whether actual cartilage damage observed after FAI induction in ovine hips occurs at the predicted, computed zone of FAI; (2) whether the extent of cartilage damage increases with ambulation time in this animal model? DESIGN: In this experimental, controlled, comparative study 20 sheep underwent unilateral FAI induction through an intertrochanteric varus osteotomy. Preoperatively sheep underwent computed tomography to generate three-dimensional models of the osseous pelvis and femur. The models were used to predict impingement zones before and after simulated varus osteotomy using range of motion (ROM) analysis. Sheep were sacrificed after 14-40 weeks of ambulation. At sacrifice cartilage was inspected and (1) location of actual damage and computed impingement zones were compared; (2) Cartilage damage was compared between short- and long ambulation groups. RESULTS: (1) The average location and the extent of peripheral and central cartilage lesions did not differ with the computed impingement zones (all P > 0.05). (2) Grades of central, posterior cartilage damage were more severe in the long-compared to the short ambulation group (2.2 ± 1.8 vs 0.4 ± 0.5; P = 0.030). CONCLUSIONS: In this experimental ovine FAI model the surgical induction of an osseous impingement conflict between the femur and acetabulum causes cartilage damage at the zone of simulated FAI.

Patients with severe slipped capital femoral epiphysis treated by the modified Dunn procedure have low rates of avascular necrosis, good outcomes, and little osteoarthritis at long-term follow-up.

AIMS: The modified Dunn procedure has the potential to restore the anatomy in hips with severe slipped capital femoral epiphyses (SCFE). However, there is a risk of developing avascular necrosis of the femoral head (AVN). In this paper, we report on clinical outcome, radiological outcome, AVN rate and complications, and the cumulative survivorship at long-term follow-up in patients undergoing the modified Dunn procedure for severe SCFE. PATIENTS AND METHODS: We performed a retrospective analysis involving 46 hips in 46 patients treated with a modified Dunn procedure for severe SCFE (slip angle > 60°) between 1999 and 2016. At nine-year-follow-up, 40 hips were available for clinical and radiological examination. Mean preoperative age was 13 years, and 14 hips (30%) presented with unstable slips. Mean preoperative slip angle was 64°. Kaplan-Meier survivorship was calculated. RESULTS: At the latest follow-up, the mean Merle d'Aubigné and Postel score was 17 points (14 to 18), mean modified Harris Hip Score was 94 points (66 to 100), and mean Hip Disability and Osteoarthritis Outcome Score was 91 points (67 to 100). Postoperative slip angle was 7° (1° to 16°). One hip (2%) had progression of osteoarthritis (OA). Two hips (5%) developed AVN of the femoral head and required further surgery. Three other hips (7%) underwent implant revision due to screw breakage or change of wires. Cumulative survivorship was 86% at ten-year follow-up. CONCLUSION: The modified Dunn procedure for severe SCFE resulted in a low rate of AVN, low risk of progression to OA, and high functional scores at long-term follow-up. The slip deformities were mainly corrected but secondary impingement deformities can develop in some hips and may require further surgical treatment. Cite this article: Bone Joint J 2019;101-B:403-414.

Usefulness of MR Arthrography of the Hip with and without leg Traction in Detection of Intra-articular Bodies.

RATIONALE AND OBJECTIVE: Although intra-articular bodies are a classic indication for MR arthrography and surgical removal, diagnostic studies are currently sparse. To assess the diagnostic performance of MR arthrography with and without leg traction in detection of intra-articular bodies in the hip joint. MATERIAL AND METHODS: The institutional hip arthroscopy data base (2009-2016: 631 hips) was retrospectively reviewed. Inclusion criteria were hips with and without intra-articular bodies and direct MR arthrography performed with and without leg traction. Twenty-one hips with intra-articular bodies constituted the "disease-positive" group. Seventy-nine randomly selected hips without intra-articular bodies constituted the "disease-negative" group. Images were reviewed independently for presence of intra-articular bodies by two blinded readers. Overall diagnosis and location of intra-articular bodies was recorded (peripheral or central). Arthroscopy served as goldstandard for diagnosis and location of intra-articular bodies. Diagnostic performance and kappa statistics of traction MR arthrography with and without traction were calculated. RESULTS: For both readers sensitivity/specificity of traction MR arthrography was 86%-95% respectively 90%-91% for overall diagnosis of intra-articular bodies and was 81%-86% respectively 90%-92%for MR arthrogrpahy without traction. For central intra-articular bodies sensitivity was higher for both readers with traction (79%-89%) than without traction (74% each). CONCLUSION: MR arthrography with and without traction of the hip is highly accurate in identifying central and peripheral intra-articular bodies. Application of traction was further useful for visualization of centrally located intra-articular bodies.

Femoral osteochondroplasty can be performed effectively without the risk of avascular necrosis or femoral neck fractures in an experimental ovine FAI model.

OBJECTIVE: The experimental induction of cam-type femoroacetabular impingement (FAI) in sheep is established. To tap the full potential of this ovine model, one should be able to perform a femoral osteochondroplasty safely. This study was based on previous cadaver experiments on the blood supply to the ovine femoral head and on the biomechanical strength of the proximal femur following offset creation. We hypothesized that offset creation in this ovine FAI model does not lead to (1) avascular necrosis (AVN) of the ovine femoral head or (2) iatrogenic femoral neck fractures and (3) can be performed effectively. DESIGN: In this experimental, controlled, prospective study nine sheep underwent unilateral FAI induction through an intertrochanteric, varus osteotomy. Seventy days following FAI induction, femoral osteochondroplasty was performed. Sheep were sacrificed after another 140 days. Radiographs, computed tomography (CT) scans and MRI were acquired. Histologic samples were stained with hematoxylin-eosin. (1) The multimodal Association Research Circulation Osseous (ARCO) classification was used for assessment of AVN. (2) Femoral neck fractures were assessed with the multimodal imaging approach. (3) Pre- and postoperative (=after sacrifice) alpha angles and femoral neck diameters were compared. RESULTS: (1) No signs for AVN according to the ARCO classification or (2) for femoral neck fractures were detected. (3) Mean alpha angles and femoral neck diameters decreased significantly (p < 0.001) superiorly by at least 30° respectively 4 mm after the offset creation. CONCLUSIONS: Femoral osteochondroplasty can be performed effectively and without the risk of AVN or femoral neck fractures in this ovine FAI model.

Patients undergoing surgical hip dislocation for the treatment of acetabular fractures show favourable long-term outcome.

AIMS: The aims of this study were to determine the cumulative ten-year survivorship of hips treated for acetabular fractures using surgical hip dislocation and to identify factors predictive of an unfavourable outcome. PATIENTS AND METHODS: We followed up 60 consecutive patients (61 hips; mean age 36.3 years, standard deviation (sd) 15) who underwent open reduction and internal fixation for a displaced fracture of the acetabulum (24 posterior wall, 18 transverse and posterior wall, ten transverse, and nine others) with a mean follow-up of 12.4 years (sd 3). RESULTS: Clinical grading was assessed using the modified Merle d'Aubigné score. Radiographic osteoarthritis was graded according to Matta. Kaplan-Meier survivorship and a univariate Cox-regression analysis were carried out using the following endpoints: total hip arthroplasty, a Merle d'Aubigné score of < 15 and/or progression of osteoarthritis. CONCLUSION: The ten-year cumulative survivorship was 82% (95% confidence interval 71 to 92). Predictors for the defined endpoints were femoral chondral lesions, marginal impaction, duration of surgery, and age of patient. Cite this article: Bone Joint J 2017;99-B:508-15.

[Bernese periacetabular osteotomy. : Indications, technique and results 30 years after the first description]. / Periazetabuläre Osteotomie nach Ganz : Indikationen, Technik und Ergebnisse 30 Jahre nach Erstbeschreibung.

The Bernese periacetabular osteotomy (PAO) is a surgical technique for the treatment of (1) hip dysplasia and (2) femoroacetabular impingement due to acetabular retroversion. The aim of the surgery is to prevent secondary osteoarthritis by improvement of the hip biomechanics. In contrast to other pelvic osteotomies, the posterior column remains intact with this technique. This improves the inherent stability of the acetabular fragment and thereby facilitates postoperative rehabilitation. The birth canal remains unchanged. Through a shortened ilioinguinal incision, four osteotomies and one controlled fracture around the acetabulum are performed. The direction of acetabular reorientation differs for both indications while the sequence of the osteotomies remains the same. This surgical approach allows for a concomitant osteochondroplasty in the case of an aspherical femoral head-neck junction. The complication rate is relatively low despite the complexity of the procedure. The key point for a successful long term outcome is an optimal reorientation of the acetabulum for both indications. With an optimal reorientation and a spherical femoral head, the cumulative survivorship of the hip after 10 years is 80-90 %. For the very first 75 patients, the cumulative 20-year survivorship was 60 %. The preliminary evaluation of the same series at a 30-year follow-up still showed a survivorship of approximately 30 %. The PAO has become the standard procedure for the surgical therapy of hip dysplasia in adolescents and adults.

Prevention of cement leakage into the hip joint by a standard cement plug during PFN-A cement augmentation: a technical note.

Medial penetration of the helical blade into the hip joint after fixation of trochanteric fractures using the proximal femur nail antirotation (PFN-A) is a potential failure mode. In low demand patients a blade exchange with cement augmentation may be an option if conversion to total hip arthroplasty is unfeasible to salvage the cut-through. This article describes a technique to avoid intraarticular cement leakage using a cement plug to close the defect in the femoral head caused by the cut-through.

Size and shape of the lunate surface in different types of pincer impingement: theoretical implications for surgical therapy.

OBJECTIVE: Acetabular rim trimming is indicated in pincer hips with an oversized lunate surface but could result in a critically decreased size of the lunate surface in pincer hips with acetabular malorientation. There is a lack of detailed three-dimensional anatomy of lunate surface in pincer hips. Therefore, we questioned how does (1) size and (2) shape of the lunate surface differ among hips with different types of pincer impingement? METHOD: We retrospectively compared size and shape of the lunate surface between acetabular retroversion (48 hips), deep acetabulum (34 hips), protrusio acetabuli (seven hips), normal acetabuli (30 hips), and hip dysplasia (45 hips). Using magnetic resonance imaging (MRI) arthrography with radial slices we measured size in percentage of the femoral head coverage and shape using the outer (inner) center-edge angles and width of lunate surface. RESULTS: Hips with retroversion had a decreased size and deep hips had normal size of the lunate surface. Both had a normal shape of the outer acetabular rim. Protrusio hips had an increased size and a prominent outer acetabular rim. In all three types of pincer hips the acetabular fossa was increased. CONCLUSION: Size and shape of the lunate surface differs substantially among different types of pincer impingement. In contrast to hips with protrusio acetabuli, retroverted and deep hips do not have an increased size of the lunate surface. Acetabular rim trimming in retroverted and deep hips should be performed with caution. Based on our results, acetabular reorientation would theoretically be the treatment of choice in retroverted hips.

Mid-term results in relation to age and analysis of predictive factors after fixation of acetabular fractures using the modified Stoppa approach.

INTRODUCTION: Data concerning outcome after management of acetabular fractures by anterior approaches with focus on age and fractures associated with roof impaction, central dislocation and/or quadrilateral plate displacement are rare. METHODS: Between October 2005 and April 2009 a series of 59 patients (mean age 57 years, range 13-91) with fractures involving the anterior column was treated using the modified Stoppa approach alone or for reduction of displaced iliac wing or low anterior column fractures in combination with the 1st window of the ilioinguinal approach or the modified Smith-Petersen approach, respectively. Surgical data, accuracy of reduction, clinical and radiographic outcome at mid-term and the need for endoprosthetic replacement in the postoperative course (defined as failure) were assessed; uni- and multivariate regression analysis were performed to identify independent predictive factors (e.g. age, nonanatomical reduction, acetabular roof impaction, central dislocation, quadrilateral plate displacement) for a failure. Outcome was assessed for all patients in general and in accordance to age in particular; patients were subdivided into two groups according to their age (group "<60yrs", group "≥60yrs"). RESULTS: Forty-three of 59 patients (mean age 54yrs, 13-89) were available for evaluation. Of these, anatomic reduction was achieved in 72% of cases. Nonanatomical reduction was identified as being the only multivariate predictor for subsequent total hip replacement (Adjusted Hazard Ratio 23.5; p<0.01). A statistically significant higher rate of nonanatomical reduction was observed in the presence of acetabular roof impaction (p=0.01). In 16% of all patients, total hip replacement was performed and in 69% of patients with preserved hips the clinical results were excellent or good at a mean follow up of 35±10 months (range: 24-55). No statistical significant differences were observed between both groups. CONCLUSION: Nonanatomical reconstruction of the articular surfaces is at risk for failure of joint-preserving management of acetabular fractures through an isolated or combined modified Stoppa approach resulting in total joint replacement at mid-term. In the elderly, joint-preserving surgery is worth considering as promising clinical and radiographic results might be obtained at mid-term.

[Posterior approaches to the acetabulum]. / Posteriore Zugänge zum Acetabulum.

Posterior approaches to the hip joint were developed by Langenbeck and Kocher in the nineteenth century. Letournel created the term Kocher-Langenbeck approach which became one of the most important approaches to the hip joint. The further extension of this approach by digastric trochanteric osteotomy and subsequently by surgical hip dislocation enables visualization of the entire hip joint which allows complete evaluation of articular joint damage, quality of reduction and confirmation of extra-articular hardware. With the increasing incidence of acetabular fractures in the elderly there is a concomitant increase of complicating factors, such as multifragmentary posterior wall fractures, dome impaction, marginal impaction and femoral head damage. These factors are negative predictors and compromise a favorable outcome after acetabular surgery. With direct joint visualization these factors can be reliably recognized and corrected as adequately as possible. Surgical hip dislocation thus offers advantages in complex posterior wall, transverse and T-shaped fractures with or without posterior wall involvement. For these fracture types surgical hip dislocation represents a standard approach in our hands.

The iliocapsularis muscle: an important stabilizer in the dysplastic hip.

BACKGROUND: The iliocapsularis muscle is a little known muscle overlying the anterior hip capsule postulated to function as a stabilizer of dysplastic hips. Theoretically, this muscle would be hypertrophied in dysplastic hips and, conversely, atrophied in stable and well-constrained hips. However, these observations have not been confirmed and the true function of this muscle remains unknown. QUESTIONS/PURPOSES: We quantified the anatomic dimensions and degree of fatty infiltration of the iliocapsularis muscle and compared the results for 45 hips with deficient acetabular coverage (Group I) with 40 hips with excessive acetabular coverage (Group II). PATIENTS AND METHODS: We used MR arthrography to evaluate anatomic dimensions (thickness, width, circumference, cross-sectional area [CSA], and partial volume) and the amount of fatty infiltration. RESULTS: We observed increased thickness, width, circumference, CSA, and partial volume of the iliocapsularis muscle in Group I when compared with Group II. Additionally, hips in Group I had a lower prevalence of fatty infiltration compared with those in Group II. The iliocapsularis muscle typically was hypertrophied, and there was less fatty infiltration in dysplastic hips compared with hips with excessive acetabular coverage. CONCLUSION: These observations suggest the iliocapsularis muscle is important for stabilizing the femoral head in a deficient acetabulum. This muscle serves as an anatomic landmark when performing a periacetabular osteotomy. Additionally, preoperative evaluation of morphologic features of the muscle can be used as an adjunct for decision making when treating patients with borderline hip dysplasia or femoroacetabular impingement.

Material properties of common suture materials in orthopaedic surgery.

Suture materials in orthopaedic surgery are used for closure of wounds, repair of fascia, muscles, tendons, ligaments, joint capsules, and cerclage or tension band of certain fractures. The purpose of this study was to compare the biomechanical properties of eleven commonly used sutures in orthopaedic surgery. Three types of braided non-absorbable and one type of braided absorbable suture material with different calibers (n=77) underwent biomechanical testing for maximum load to failure, strain, and stiffness. All samples were tied by one surgeon with a single SMC (Seoul Medical Center) knot and three square knots. The maximum load to failure and strain were highest for #5 FiberWire and lowest for #0 Ethibond Excel (p<0.001). The stiffness was highest for #5 FiberWire and lowest for #2-0 Vicryl (p<0.001). In all samples, the failure of the suture material occurred at the knot There was no slippage of the knot in any of the samples tested. This data will assist the orthopaedic surgeon in selection and application of appropriate suture materials and calibers to specific tasks.

Surgical dislocation of the hip for the fixation of acetabular fractures.

Surgical dislocation of the hip in the treatment of acetabular fractures allows the femoral head to be safely displaced from the acetabulum. This permits full intra-articular acetabular and femoral inspection for the evaluation and potential treatment of cartilage lesions of the labrum and femoral head, reduction of the fracture under direct vision and avoidance of intra-articular penetration with hardware. We report 60 patients with selected types of acetabular fracture who were treated using this approach. Six were lost to follow-up and the remaining 54 were available for clinical and radiological review at a mean follow-up of 4.4 years (2 to 9). Substantial damage to the intra-articular cartilage was found in the anteromedial portion of the femoral head and the posterosuperior aspect of the acetabulum. Labral lesions were predominantly seen in the posterior acetabular area. Anatomical reduction was achieved in 50 hips (93%) which was considerably higher than that seen in previous reports. There were no cases of avascular necrosis. Four patients subsequently required total hip replacement. Good or excellent results were achieved in 44 hips (81.5%). The cumulative eight-year survivorship was 89.0% (95% confidence interval 84.5 to 94.1). Significant predictors of poor outcome were involvement of the acetabular dome and lesions of the femoral cartilage greater than grade 2. The functional mid-term results were better than those of previous reports. Surgical dislocation of the hip allows accurate reduction and a predictable mid-term outcome in the management of these difficult injuries without the risk of the development of avascular necrosis.

[Imaging in developmental dysplasia of the hip]. / Bildgebende Diagnostik der Hüftdysplasie.

Modern imaging techniques are an invaluable tool for assessing pathomorphological changes of the hip. Thorough diagnostic analysis and therapeutic decision making mainly rely on correct interpretation of conventional radiographic projections as well as more modern techniques, including magnetic resonance arthrography. This article gives an overview of the imaging techniques that are routinely used for assessing pathological conditions of the hip, with a special focus on diagnostic findings in developmental dysplasia of the hip as well as in femoroacetabular impingement.

Femoral morphology differs between deficient and excessive acetabular coverage.

UNLABELLED: Structural deformities of the femoral head occurring during skeletal development (eg, Legg-Calvé-Perthes disease) are associated with individual shapes of the acetabulum but it is unclear whether differences in acetabular shape are associated with differences in proximal femoral shape. We questioned whether the amount of acetabular coverage influences femoral morphology. We retrospectively compared the proximal femoral anatomy of 50 selected patients (50 hips) with developmental dysplasia of the hip (lateral center-edge angle [LCE] < or = 25 degrees ; acetabular index > or = 14 degrees ) with 45 selected patients (50 hips) with a deep acetabulum (LCE > or = 39 degrees ). Using MRI arthrography we measured head sphericity, epiphyseal shape, epiphyseal extension, and femoral head-neck offset. A deep acetabulum was associated with a more spherical head shape, increased epiphyseal height with a pronounced extension of the epiphysis towards the femoral neck, and an increased offset. In contrast, dysplastic hips showed an elliptical femoral head, decreased epiphyseal height with a less pronounced extension of the epiphysis, and decreased head-neck offset. Hips with different acetabular coverage are associated with different proximal femoral anatomy. A nonspherical head in dysplastic hips could lead to joint incongruity after an acetabular reorientation procedure. LEVEL OF EVIDENCE: Level IV, retrospective comparative study. See the Guidelines for Authors for a complete description of levels of evidence.

Accuracy considerations in navigated cup placement for total hip arthroplasty.

Computer assisted orthopaedic surgery (CAOS) technology has recently been introduced to overcome problems resulting from acetabular component malpositioning in total hip arthroplasty. Available navigation modules can conceptually be categorized as computer tomography (CT) based, fluoroscopy based, or image-free. The current study presents a comprehensive accuracy analysis on the computer assisted placement accuracy of acetabular cups. It combines analyses using mathematical approaches, in vitro testing environments, and an in vivo clinical trial. A hybrid navigation approach combining image-free with fluoroscopic technology was chosen as the best compromise to CT-based systems. It introduces pointer-based digitization for easily assessable points and bi-planar fluoroscopy for deep-seated landmarks. From the in vitro data maximum deviations were found to be 3.6 degrees for inclination and 3.8 degrees for anteversion relative to a pre-defined test position. The maximum difference between intraoperatively calculated cup inclination and anteversion with the postoperatively measured position was 4 degrees and 5 degrees, respectively. These data coincide with worst cases scenario predictions applying a statistical simulation model. The proper use of navigation technology can reduce variability of cup placement well within the surgical safe zone. Surgeons have to concentrate on a variety of error sources during the procedure, which may explain the reported strong learning curves for CAOS technologies.

Hip2Norm: an object-oriented cross-platform program for 3D analysis of hip joint morphology using 2D pelvic radiographs.

We developed an object-oriented cross-platform program to perform three-dimensional (3D) analysis of hip joint morphology using two-dimensional (2D) anteroposterior (AP) pelvic radiographs. Landmarks extracted from 2D AP pelvic radiographs and optionally an additional lateral pelvic X-ray were combined with a cone beam projection model to reconstruct 3D hip joints. Since individual pelvic orientation can vary considerably, a method for standardizing pelvic orientation was implemented to determine the absolute tilt/rotation. The evaluation of anatomically morphologic differences was achieved by reconstructing the projected acetabular rim and the measured hip parameters as if obtained in a standardized neutral orientation. The program had been successfully used to interactively objectify acetabular version in hips with femoro-acetabular impingement or developmental dysplasia. Hip(2)Norm is written in object-oriented programming language C++ using cross-platform software Qt (TrollTech, Oslo, Norway) for graphical user interface (GUI) and is transportable to any platform.

[The foundations of computer assisted surgery]. / Grundlagen der computerassistierten Chirurgie.

Using navigation systems in general orthopaedic surgery and, in particular, knee replacement is becoming more and more accepted. This paper describes the basic technological concepts of modern computer assisted surgical systems. It explains the variation in currently available systems and outlines research activities that will potentially influence future products. In general, each navigation system is defined by three components: (1) the therapeutic object is the anatomical structure that is operated on using the navigation system, (2) the virtual object represents an image of the therapeutic object, with radiological images or computer generated models potentially being used, and (3) last but not least, the navigator acquires the spatial position and orientation of instruments and anatomy thus providing the necessary data to replay surgical action in real-time on the navigation system's screen.