Computer navigation is a highly sophisticated tool in orthopedic surgery for component placement in total hip arthroplasty (THA). In order to apply it adequately it is of upmost importance that the targets the surgeon is trying to hit are well-defined. This concept considers all four component orientations: cup inclination (cIncl) and anteversion (cAV), stem antetorsion and neck-to-shaft angle. The optimising goal in this concept is maximising the size of the cSafe-Zone. A computerised 3D- model of a total hip prosthesis was used to systematically analyse all combinations of component orientations in automatised batch runs. Component orientations were varied for cup inclination, cup anteversion, neck antetorsion and neck inclination.Introduction
Methods
Lewinnek's Safe-Zone gives recommendations only for cup placement in total hip arthroplasty while the orientation of the neck isn't considered. Furthermore the criteria for cup placement are not clearly defined and the ranges for cup orientation are considerably large. This study introduces new recommandations for the combined placement of both total hip components, when both, cup and stem, are considered. This defines the new dynamic combined safe-zone (cSafe-Zone) which gives clear directions for the optimal combined orientation of both components in order to maximize the intended range of movement (iROM) while reducing the risk for prosthetic impingement and dislocation. The combined safe-zone outlines the area that encloses all component orientations that achieve the predefined iROM without prosthetic impingement. A computerized 3D-model of a total hip prosthesis was established that does systematically test all design parameters semi-automatically in order to identify those component positions that fulfill the predefined conditions. The analysis was carried out for straight stems, anatomic stems and short stems. The iROM is composed of basic movements like flexion/extension, internal/external rotation, ab/adduction and combination of these movements that the patient should reach and that are commonly accepted as physiologic hip movements. The orientation of the cup was varied between 20° and 70° of inclination and −10° of retro- to 40° anteversion. Stem antetorsion was tested from −10° retro- to 40°-antetorsion and CCD-angle from 110° to 150°. Head-size and head/neck ratio were additional parameters.Introduction
Material and Methods
Dislocation due to suboptimal cup positioning is a devastating complication in the early phase after total hip arthroplasty. Malpositioning can also result in other mechanical complications like subluxation, edge loading, increased debris, surface damage or squeaking in ceramic-on-ceramic hips. Preventing at least some of these complications in younger and more active patients is of paramount interest for the individual patient and for the society since optimized component orientation is an important determinant to reduce such risks and to further increase longevity of the implant. This study reports on two new surgical instruments that help the orthopedic surgeon to manually place both components within the optimized combined safe-zone (cSafe-Zone). More than 900 minimal-invasive total hip arthroplasties (MIS-THA) have been performed between 2007 and 2015 in our institution using the minimal-invasive direct anterior approach (DAA) on an orthopedic table with foot holder. Cups were implanted applying the “stem-first” surgical technique i.e. the prosthetic stem dictates the orientation of the socket depending on the prosthesis design. A system-specific trial head which indicates the prosthesis-specific relative orientation of cup and stem and a modified cup impactor were used to finally seat the definitive acetabular socket manually during trial stem reduction while fully visually controlling the optimal orientation of the cup during impaction. This surgical technique drives both components into their optimal relative positions according to the combined version and the combined safe-zone concept in total hip arthroplasty.Introduction
Material and Methods
Two principal targets are dominating the spectrum of goals in total knee arthroplasty: first of all the orthopedic surgeon aims at achieving an optimal pain-free postoperative kinematic motion close to the individual physiologic range of the individual patient and secondly he aims for a concurrent high ligament stability within the entire range of movement in order to establish stability for all activities of daily living. This study presents a modified surgical procedure for total knee replacement which is ligament-controlled in order to put both component into the “ligamentous frame” of the patients individual kinematics. The posterior femero-condylar index (PFC-I) is defined as being the posterior condylar offset divided by the distal antero-posterior diameter on a lateral radiograph. After careful preoperative planning the positions and orientations of the osteotomies is controlled intraoperatively via ligamentous guidance. Anterior and distal femoral osteotomy are planned on antero-posterior and lateral radiographs considering intramedular and mechanical axes as well as the orientation of the posterior condyles. Osteotomies are carried out in a stepwise fashion, starting with the anterior femoral osteotomy followed by the distal femoral osteotomy as planned. Then the extension gap is finalized by tensioning the ligaments and “top-down” referencing the level of the tibial osteotomy. After rotating the femur into the 90°-flexion position the flexion gap is finalized by referencing the level of the posterior condyle osteotomy in a “bottom-up” fashion to the tibial osteotomy. Hence, this technique determines the size of the femoral component with the last osteotomy. It likewise respects the new, ACL-lacking ligamentous framework and it drives the prosthetic components to fit into the new ligamentous envelope to follow the modified kinematics.Introduction
Methods
Dislocation is still one of the major complications in total hip arthroplasty. Among other factors, it is important to maximize the intended range of movement (iROM) in order to reduce the risk for prosthetic impingement and to prevent edge loading in order to avoid surface damage and squeaking. Therefore, both components should be positioned in accordance to the new combined safe-zone for correct combined version and inclination aiming for an optimal relative orientation of both components. This study shows how this optimal combined orientation of both components can be determined for a specific total hip prosthetic system and how the result can be transferred to surgery and accomplished intraoperatively using minimal-invasive stem-first technique. 829 minimal-invasive total hip arthroplasties have been performed from 2007 to 2013 in our institution. In all of these surgeries a minimal-invasive direct anterior approach (DAA) was applied. All patients were positioned supine on a proprietary orthopedic table. In 168 cases a system-specific mechanical aiming device was used intraoperatively in order to control the combined version and inclination according to the specific safe-zone by orienting the prosthetic socket in relation to the prosthetic stem. This is called “stem-first technique” meaning that the prosthetic stem dictates the orientation of the socket. It does not mean the sequence of implantation. It's specific advantage is that the stem redirects the socket if it's position is modified. It equally applies to standard, anatomic (Fig. 1) or short stems (Fig. 2). The socket orientation is not primarily referred to bony landmarks of the pelvis but to the neck of the stem. Nevertheless the surgeon has to look for sufficient fixation of the socket within the acetabular bone. Leg length was measured in the surgical field additionally and prosthetic offset was adjusted according to the preoperative planning. All other patients were operated on in our standard minimal-invasive DAA-procedure. The majority of the patients suffered from osteoarthritis, their mean age was 68 years.Introduction:
Material and Methods:
After total hip arthroplasty, dislocation is one of the most frequent serious early complications. This occurs in part due to impingement (catching and leverage of the neck-cup on the inlay/cup border). Impingement may also negatively impact long-term outcomes. A preliminary model for an optimised hip endoprosthesis system was developed to offer a mechanical solution to avoid impingement and dislocation. A computer-supported range of motion simulation using parameters of cup anteversion and inclination as well as torsion and CCD shaft angle was then performed to localise areas of anterior and posterior impingement of typical acetabular cups.Introduction
Materials and Methods
