Opening-wedge High Tibial Osteotomy (HTO) has been shown to be an effective procedure to treat mild to moderate osteoarthritis of the medial compartment of the knee in active individuals. It has also become a mandatory surgical adjunct to articular cartilage restoration when there is preoperative mal-alignment. However, its efficacy is directly correlated with the accuracy of the correction, which must be within 3° of the preoperative target. Achieving this goal is a significant challenge with conventional techniques. Therefore, computer-assisted navigation protocols have been developed; however, they do not adequately address the technical difficulties associated with this procedure. We present an integrated solution dedicated to the opening-wedge HTO. Advantages to the technique we propose include: 1) a minimum number of implanted bone trackers, 2) depth control of the saw, 3) improved 3-D accuracy in the location of the lateral tibial hinge, and 4) micrometric adjustment of the degree of correction. The proof of concept has been completed on all six specimens. The following key points have been validated: a) Compatibility with a minimally-invasive (5–6 cm) surgical incision b) The compact navigation station can be placed close to the operative field and manipulated through a sterile draping device c) Only two trackers are necessary to acquire the required landmarks and to provide 3-D control of the correction. These can be inserted within the surgical wound without any secondary incisions d) The optimised guide accurately controlled the external tibial hinge in all six cases e) The implant cavity could be milled effectively f) The distractor used to complete the desired realignment maintained stability of the distraction until final fixation with the PEEK implant g) The PEEK implant could be fixed to the tibia with excellent stability in a low-profile fashion. The solution presented here has the potential to help surgeons perform a medial opening-wedge HTO more safely and accurately. This will likely result in an increase in the number of HTOs performed for both isolated medial compartment osteoarthritis as well as for lower extremity realignment in association with cartilage restorative procedures.

Soft tissue gaps created in total knee replacement rely on the creation of symmetrical spaces that accommodate prosthetic implants. We studied a new custom surface registration protocol in a computer navigation system to accurately and precisely measure these gaps. In eight cadaver lower extremities, gaps were measured from the proximal tibial cut surface to the registered most distal surfaces of the medial and lateral femoral condyles, measured from 0 to 120 degrees. The computer measurement was compared against metrology spacers precise to 200 microns. Tensor reproducibility was assessed using a typical teeter-toter tensor in four specimens with cruciate retained and four with sacrificing technique.

Generalised MANOVA tests were used for assessment of means of repeated measures involving the three separate experiments. There was no difference between the measurements obtained using computer navigation compared to the metrology spacers in one specimen including the re-registration group (P = NS, Beta = 0.9). The sagittal position of the knee (Flexion/Extension) did affect the magnitude of the measurements obtained. (P=.001) For comparison, descriptive statistics of spacer block versus navigation measure revealed for the medial compartment measurement, a mean (n=200) of 0.006 mm (SD: 0.32 mm) and lateral compartment measure (n=200) of 0.12 mm (SD: 0.41 mm). The projected maximum error was 1.0 mm capturing 100% of values to 90 degrees. The re-registration repeated measures experiment varied as a function of knee flexion and the repetition number. Descriptive statistics for comparison revealed a mean medial compartment measure (n=200) of 0.24 mm (SD: 0.54 mm) and lateral compartment measure(n=200) of 0.01 mm(SD: 0.42 mm).

The tensor study compared the ability of the surgeon to produce a consistent gap measure over eight separate trials. Hypothesis testing revealed significant differences as a function of degree of flexion, order of testing (with later tests having greater gaps), and the specimen being measured (P<.001, P<.001, and P<.001).

The overall conclusion of the block studies was that the computer system was accurate to at least one millimeter for measuring the gaps of the knee. The tensor study demonstrated stretching or permanent strain of the ligaments, significant differences between the angles of flexion and between the individual specimens. This is to say that each specimen was unique with variability of measurements through the range of motion.

In TKA, prosthetic femoral and tibial implants must be symmetrically placed and matched in the mechanical axis and the ligament gaps must be correctly balanced. The collateral ligaments are the key guide, as they arise from the epicondyles of the distal femur, are perpendicular to the AP axis of Whiteside, and are coincident with the transtibial axis of the proximal tibial surface. A perpendicular bisection of the transtibial axis creates the AP axis of the tibia which is coincident in space with the AP axis of Whiteside (Berger). Measured distal femoral resection targets including TEA, AP axis of Whiteside, and 3 degrees external to the posterior condylar axis works because the stout posterior cruciate ligament limits laxity in flexion, allowing for the anatomical variation of these landmarks to be accommodated. The Insall, Ranawat gap balancing methods work to balance the knee in flexion, often matching the results of a measured resection, but guaranteeing a symmetrically balanced flexion gap. Distal femoral internal rotation can result if the medial collateral is over-released, but experience has shown this not to be a problem if the gaps are well balanced. Tibial tray position must be placed coincident with the AP axis of the tibia, which also is coincident with Akagi's line (line from medial margin of patellar tendon to center of the posterior cruciate ligament). The surgeon can make a line from the AP axis of Whiteside to the anterior tibial which matches the AP tibial axis.

Major aspects on long-term outcome in Total Knee Arthroplasty are the correct alignment of the implant with the mechanical load axis, the rotational alignment of the components as well as good soft tissue balancing. To reduce the variability of implant alignment and at the same time minimise the invasiveness different computer assisted systems have been introduced.

To achieve accuracy as high as those of a robotic system but with a pure mechanically adjustable cutting block, the Exactech GPS system has been developed. The new concept comprises a seamlessly planning and navigation screen with an integrated optical tracking system for fast and accurate acquisition and verification of anatomical landmarks within the sterile field as well as a tiny cutting guide for accurate transfer of the planned bone resections.

Using a conventional screwdriver the cutting block could be accurately aligned with the planned resection by controlling the current position of the cutting block on the navigation screen. To save time, to maximise the ease of use and to minimize the surgeon's mental workload during adjustment, a smart screwdriver (SSD) has been developed being able to automatically adjust the screws.

The basic idea of the smart screwdriver is to have a system providing an automatic transfer of the planned data to the cutting guide similar to a robotic system, but with the actuators separated from the kinematic. The use of the SSD is as simple as follows: After planning of the intervention and rigid fixation of the cutting guide on the bone, the surgeon simply connects sequentially the screwdriver to all screws of the cutting guide.

To further maximise the ease of use and to avoid a mix-up of different screws, an identification means has been integrated into the positioning screws as well as into the smart screwdriver. For an automated identification of the screws different technologies have been analysed as position tracking, optical recognition or wired/wireless electronics.

A first prototype without screw identification has been used successfully on 4 cadaver knees. All guide positions could be adjusted automatically using the SSD. However, the absence of screw identification required that the surgeon follows indications given by the computer to turn screws sequentially.

A second prototype of the smart screwdriver has successfully been built up and is able to identify the different positioning screws in less than 1s with high reliability. The identification is realised as inductive coupling of different small resonance circuits that are integrated into the screw heads and the screwdrivers tip.

To adjust the cutting guide from neutral to the planned position, the screws have to be adjusted by 5 mm in average. The rotational speed of the current SSD implementation is 2 rounds per second, resulting in a mean time of about 3.5 s for each screw adjustment. The rotational accuracy of the screwdriver is ±5°. Taking into account a thread of the positioning screws of 0.7 mm, the theoretical translational error is about ±0.01 mm. Looking at the angular accuracy, the maximum distance of the screws of the current setup of the cutting block of 15 mm results in an angular error of less than ±0.05°.

This report reviews the long-term results of treating acetabula with unusually severe problems, such as pelvic discontinuity or major column loss after failed total hip arthroplasty (THA) and reconstruction problems.

Loss of acetabular bone stock results from removal of bone during the original procedure, prosthetic failure, and osteolysis. In massive structural failure, the acetabular rim, quadrilateral plate, and associated columns become deficient. At worst, this may be combined with pelvic discontinuity and disruption of the ilium and ischium. Prosthetic protrusio may result from fixation loss and be associated with scarring of the femoral vessels, femoral nerve, ureter and bowel. A variety of implants has been used to in ace-tabular reconstruction. The results are often poor because of insufficient bone stock to support the implant.

In a consecutive series of 251 THA revisions done between 1988 and 1996, 17 patients were treated for major pelvic column loss, pelvic discontinuity or both.

In five patients, a posterolateral approach without trochanteric osteotomy was used. The extensile triradiate approach with ilioinguinal extension was used in 12 patients in whom severe prosthetic protrusio increased the risk of intrapelvic iatrogenic injury. A long anterior column pelvic plate was applied. A posteriorly placed AO 4.5-mm pelvic reconstruction plate with 10 to 12 holes was used in nine cases of pelvic discontinuity and in five cases of posterior column bone loss. This plate extended from the most inferior extent of the ischium across the wall of the posterior column to a point high on the ilium. Anterior column fixation was done in eight of nine cases of pelvic discontinuity and all three cases of anterior column deficiency. This called for an 8 to 12-hole 3.5-mm AO pelvic reconstruction plate that extended from the pubic symphysis across the pelvic rim. This spanned the anterior column defect, ranging from 4 cm to 8 cm, to the medial wall of the ilium.

Bulk allograft was used in 16 of the 17 patients. The patient in whom allograft was not used had pelvic discontinuity following pelvic irradiation. Whole pelvic acetabular transplants were used in seven with severe bone loss or following resection for chondrosarcoma and the other for pigmented or villonodular synovitis. Posterior segmental acetabular allograft was used in two cases of posterior column absence. Femoral heads were used in two posterior column defects, three pelvic discontinuities with anterior column defect, and two anterior column defects. Acetabular components were cemented in six of seven whole bulk ace-tabular transplants, six of nine pelvic discontinuities and two anterior column defects.

Cemented implants were classified as loose if there was a complete radiolucent line at the bone cement interface, measurable component migration or measurable change in position. Uncemented acetabular components were considered loose if component migration had occurred or screws had broken. Pelvic plates were considered loose if there was measurable migration or change in plate position or if fixation screws had backed out or broken.

Radiographic union was considered present when bridging callus or trabecular bone was visible across the discontinuity site. Junctional healing was considered probable when radiographs did not show obvious signs of failure. Grafts were considered unhealed if there was obvious displacement, bone gaps or hardware breakage.

Seven of the nine patients with pelvic discontinuity had late evidence of healing of the fracture and allograft consolidation. One underwent removal of the graft at three weeks after developing acute postoperative infection: early junctional healing of a whole bulk acetabular allograft required an osteotomy to break up the interface. Another patient, who underwent removal of the graft and implant at three months for chronic infection, had consolidation of a whole bulk ace-tabular allograft. One patient underwent revision of a pressfitted acetabular component at 60 months, and the pelvic discontinuity was solidly united. In a fourth patient, explored at 124 months for loosening of a cemented cup, there was near complete dissolution of the graft posterior acetabular wall and a loose posterior pelvic plate. In a patient with pelvic discontinuity after radiation therapy for uterine carcinoma, satisfactory healing of the pelvic discontinuity was confirmed at 32 months, when excisional arthroplasty for late chronic infection followed urinary sepsis.

Seven patients had major column loss with severe cavitary defects. Consolidation of the allograft was noted in all seven within the first 12 months of follow-up.

Revision (47%) was required for infection in three patients, implant loosening in four, and recurrent implant dislocation in one. The four loose cups were revised to a cemented all-polyethylene component. All four implants had been placed on less than 50% host bone. None of the four has required subsequent revision.

Dislocation postoperatively occurred in eight patients. In six, the extensile triradiate approach had been used. This approach led to dislocation in 50%. The main reasons for using the extensile triradiate approach were to avoid catastrophic injuries by direct exposure of vital structures and to allow stable anterior column plate fixation. In that no neurovascular injuries occurred and stable durable allograft consolidation and healing of pelvic discontinuity took place, these goals were largely met.

Three patients developed late sciatic palsy. In one, plaster immobilisation had possibly caused direct pressure over the fibular head and led to chronic peroneal palsy. The other two underwent additional exploration of the sciatic nerve for late entrapment caused by migration of screws from the posterior column plate. Two patients developed bladder infections postoperatively. Another developed superficial phlebitis of the lower leg.

Acetabular revision for loosening was necessary in three of seven cementless implants, while only two of 10 cemented implants failed. The acetabular component should be cemented into the allograft when more than 50% of the prosthetic interface is non-viable.

Virtually all graft material, including dense cortical grafts, may ultimately fail if used for implant fixation. Patients should be told about the inevitable risks. However, techniques used led to stable healing of the pelvic discontinuity in most cases. Long pelvic plates that securely stabilise the pelvis and allografts carefully opposed to host bone may explain the relative success in this series.

Chronic ligament insufficiency in total knee arthroplasty is associated with extension/flexion imbalance, late rupture of the posterior cruciate ligament (PCL), excessive joint line elevation and PCL insufficiency.

To solve the ligament balance problem, designated anatomical ‘cookbook’ bone cuts are used. Cutting ligaments affects flexion and extension gaps differently, and saving the PCL makes flexion gap adjustment difficult.

In ligamentous releases, the extension gap is affected by release of pes tendons, semimembranosus, iliotibial tract, biceps tendon, gastrocnemius tendon, and popliteus. Extension and flexion gaps are affected by release of the medial and lateral capsular ligaments, superficial medial collateral ligament, and lateral collateral ligament. The 50% rule states that flexion increases 50% more than extension with release.

Revision of all implants is usually needed. A liner exchange seldom works. Flexion/extension balance remains the critical problem. Modular revision implants are critical for correction of the gaps.

This paper reviews the causes of chronic instability after total hip arthroplasty (THA).

The overall reported incidence varies from 0.5% to 9.5%. At 2% to 6%, the incidence following primary THA is higher with a posterior approach than with an anterior approach (0.5% to 3%). The incidence is reported to be as high as 22% after revision THA and 50% after extensile triradiate approach for pelvic discontinuity.

Inadequate soft tissue lengthening, damaged abductors and nonunion of trochanteric osteotomy are known to predispose patients to chronic instability after THA. Elderly women are particularly susceptible. Poor patient compliance is also a cause.

Surgical technique is also a factor. The lateral decubitus position often causes flattening of the lumbar lordosis, leading to potential cup retroversion. Over 90% of all dislocations are posterior, and disruption of external rotators and capsular damage should be repaired if possible. The optimal implant position appears to be 40° TO 45° of abduction, 15° to 20° of femoral anteversion, and 20° to 30° of cup flexion. Elevation of the hip centre weakens abductor pull, causing instability. Because a reduced femoral offset causes potential instability, this should be measured preoperatively to make sure that the stem can provide adequate offset. It may be necessary to add a thicker liner to increase the offset.

Prosthetic factors which play a role in chronic instability include the use of smaller femoral heads, thick necked stems and heads with skirts. A larger femoral head increases stability simply by increasing the radian about the hip centre, increasing the potential range of motion. Extended posterior wall-adds improve the range of motion, and consequently the stability. However, there are fears that their use may increase the incidence of impingement and/or lead to increased wear. Skirted femoral heads impinge on the liner, limiting movement, and their use should be avoided in most cases of instability.

Femoral stem offset relates to the neck shaft angle and the effective hip centre/shaft axis length or offset. It is easier to increase offset with lower neck shaft angle than to lengthen the leg. Because a bell curve is used in the design of femoral stems, many prosthetic systems lack adequate offset, especially when larger stems (48 mm to 52 mm) are used.

In earlier prosthetic designs, bulk was added to the necks to eliminate stem breakage. In certain stems, the way in which dimensions were scaled meant the neck dimensions of larger prostheses were disproportionately big. We stopped using Depuy Stability stems sizes 16 mm and 18 mm because of this. Thornberry et al have shown that a circulotrapezoidal neck design is the best shape and leads to the least impingement. They have also shown that increasing the width of the chamfer of the acetabular liner rim improves the range of motion.

In treating early instability (occurring less than 30 days postoperatively) most authors recommend bracing for six to eight weeks and warning patients severely about the long-term potential of redislocation. In cases of chronic instability (occurring more than 30 days postoperatively) all potential problems must be explored: these include soft tissue laxity, cup retroversion, inadequate offset, surgical approach, etc. In managing multiple dislocation, the use of extended immobilisation is less desirable although patients who have undergone revision have been subjected to a great deal of soft tissue dissection and potentially should be braced for up to 12 months. If the cause is correctable-malpositioning, soft tissue laxity or bony impingement – treatment is likely to be successful in 85% of cases. However, if the implants are in good position, the ‘bloodless revision’ (Fehring) has less than 50% chance of succeeding. The implication is that an extended posterior wall liner, longer modular femoral head, and soft tissue reconstruction are not going to work in the majority of cases.

Designed by Noiles, the J& J SROM constrained acetabular liner uses a polyethylene capture mechanism that is secured by two additional screws. The pullout strength of this device is 1 350 N but torque required (lever-out strength) diminishes to 17.3 N.m for a 28-mm head. With a 32 mm head, 105° of flexion was obtained (while the normal hip needs up to 113° for usual flexion). Following up 21 patients with this implant for over two years, Anderson et al found redislocation in 29%. The only causative factor identified was an abduction angle of more than 70°. However, there were no cases of implant loosening of this device. Prevention of loosening was one of the design goals in using a ‘softer’ locking mechanism. Dislodgement of the liner requires immediate re-operation.

The Osteonics constrained liner cup has a dual socket. The inner socket has a polished chrome surface manufactured fit to the outer socket. It fits a 22 mm or 28 mm head, and has a locking ring identical to the bipolar implant that holds the head in place. The implant can be snap-fitted into a 52-mm or larger Osteonics cup. This liner can also be cemented into another metal-backed liner. Goetz et al evaluated 56 cases, in 10 of which this implant had been cemented and in 46 lock-fitted in appropriately matched metal shells. In one case, the cemented constrained liner had separated from the metal shell. None of the constrained liners had separated from the metal shells, but one shell had loosened.

There are many similar constrained acetabular liners. The choice is between a ‘locked’ liner that can never separate and a ‘softer’ lock that may protect fixation of the cup.