Fernando Fonseca

Automatic Camera Calibration Applied to Medical Endoscopy

The paper proposes a new calibration algorithm for cameras with lens distortion, that uses a single image of a planar chessboard pattern acquired in general position. The radial distortion is modeled using the first order division model, and the method provides a closed form estimation of the intrinsic parameters and distortion coefficient. The experimental evaluation shows that the calibration accuracy is comparable to state-of-the-art algorithms requiring multiple input images. We believe that our approach is particularly well suited for the the calibration of medical endoscopes in computer aided surgery. Since the lens is mounted on the camera before each usage in the OR, the calibration procedure must be performed by the clinical practitioner with minimum effort. We solve this problem by proposing a fully automatic procedure that requires no human intervention other than acquiring a single calibration image.

The influence of different tibial stem designs in load sharing and stability at the cement–bone interface in revision TKA

Total Knee Arthroplasty (TKA) changes mechanical loading of the knee joint. Bone loss in the tibia is commonly encountered at the time of the revision TKA. Restoration of lost bone support and joint stability are the primary challenges in revision TKA. Normally, these defects are treated with non-living structures like metallic augments or bone grafts (autografts or allografts). Alone, neither of these structures can provide the initial support and stability for revision implants. In the latter, the use of intramedullary stems can provide the necessary load sharing and protect the remaining host bone and graft from excessive stress, increasing component stability. The purpose of this study was to evaluate comparatively load sharing (cortical rim, cancellous bone and stem) and stability at the cement-bone interface under the tibial tray induced by the use of cemented and press-fit tibial component stem extensions. Furthermore the study of the desirable option in cases where the bone defect is cavitary (cancellous bone defect contained by an intact cortical rim) or uncontained bone defect (bone loss involving the supporting cortical rim) was carried out. Because in vitro evaluation of these biomechanical parameters is difficult we used finite element (FE) models to overcome this. The biomechanical results suggest an identical behaviour in case of cavitary defects for both types of stems assessed. In the case of uncontained defect treated with bulk allografts the cemented stem may be a prudent clinical option.

Finite Element and Experimental Cortex Strains of the Intact and Implanted Tibia

Finite Element (FE) models for the simulation of intact and implanted bone find their main purpose in accurately reproducing the associated mechanical behavior. FE models can be used for preclinical testing of joint replacement implants, where some biomechanical aspects are difficult, if not possible, to simulate and investigate in vitro. To predict mechanical failure or damage, the models should accurately predict stresses and strains. Commercially available synthetic femur models have been extensively used to validate finite element models, but despite the vast literature available on the characteristics of synthetic tibia, numerical and experimental validation of the intact and implant assemblies of tibia are very limited or lacking. In the current study, four FE models of synthetic tibia, intact and reconstructed, were compared against experimental bone strain data, and an overall agreement within 10% between experimental and FE strains was obtained. Finite element and experimental (strain gauge) models of intact and implanted synthetic tibia were validated based on the comparison of cortex bone strains. The study also includes the analysis carried out on standard tibial components with cemented and noncemented stems of the P.F.C Sigma Modular Knee System. The overall agreement within 10% previously established was achieved, indicating that FE models could be successfully validated. The obtained results include a statistical analysis where the root-mean-square-error values were always <10%. FE models can successfully reproduce bone strains under most relevant acting loads upon the condylar surface of the tibia. Moreover, FE models, once properly validated, can be used for preclinical testing of tibial knee replacement, including misalignment of the implants in the proximal tibia after surgery, simulation of long-term failure according to the damage accumulation failure scenario, and other related biomechanical aspects.

Simulation of Physiological Loading in Total Hip Replacements

The determination of biomechanical force systems of implanted femurs to obtain adequate strain measurements has been neglected in many published studies. Due to geometric alterations induced by surgery and those inherent to the design of the prosthesis, the loading system changes because the lever arms are modified. This paper discusses the determination of adequate loading of the implanted femur based on the intact femur-loading configuration. Four reconstructions with Lubinus SPII, Charnley Roundback, Muller Straight and Stanmore prostheses were used in the study. Pseudophysiologic and nonphysiologic implanted system forces were generated and assessed with finite element analysis. Using an equilibrium system of forces composed by the Fx (medially direction) component of the hip contact force and the bending moments Mx (median plane) and My (coronal plane) allowed adequate, pseudo-physiological loading of the implanted femur. We suggest that at least the bending moment at the coronal plane must be restored in the implanted femur-loading configuration.

Biomechanical evaluation of different reconstructive techniques of proximal tibia in revision total knee arthroplasty: An in-vitro and finite element analysis

BACKGROUND Bone loss and subsequent defects are often encountered in revision total knee arthroplasty. In particular, when the cortical rim of proximal tibia is breached, the surgical decision on the reconstructive options to be taken is challenging due to the variety of defects and the lack of data from clinical or experimental studies that can support it. The purpose of this study is to assess how different reconstructive techniques, when applied to an identical defect and bone condition, can be associated to dissimilar longevity of the revision procedure, and the role of a stem in this longevity. METHODS Proximal cortex strains and implant stability were measured in ten reconstructive techniques replicated with synthetic tibiae. The cancellous bone strains under each construct were assessed with finite element models which were validated against experimental strains. FINDINGS The measured strains and stability showed that the proximal cortex is not immune to the different reconstructive techniques when applied to an identical defect. The largest cancellous strain differences between modular and non-modular techniques indicate a distinct risk between reconstructive techniques, associated to the supporting capacity of cancellous bone at long term. INTERPRETATION The main finding of the present study is the observation that modular augments increases, on a long term basis, the potential risk of bone resorption relative to the non-modular techniques. In addition, the use of a press-fit stem in the scope of non-modular techniques can lead to improved stability and load transfer, which can contribute positively to the life expectancy of these techniques.

A new press-fit stem concept to reduce the risk of end-of-stem pain at revision TKA: A pre-clinical study

PURPOSE Revision total knee arthroplasty presents numerous technical challenges, with lower patient outcomes compared with those obtained in primary surgery. Extended stems have been used in revision total knee arthroplasty to improve component alignment and fixation. Hybrid fixation with cemented tibial tray and press-fit stem has shown good results. One of the disadvantages of this technique is pain related to the presence of a cementless diaphyseal engaging stem, often designated as end-of-stem pain. Patients with this pain have reported a decrease in overall satisfaction, as well as demonstrate a lower clinical outcome score. Clinical findings suggest that stem material and design are important factors in the development of end-of-stem pain. Therefore, a question can be raised: can a novel press-fit stem concept minimize bone strain changes at the stem tip? The hypothesis here considered lies upon the fact, that if periosteal cortex strain changes are minimized at the stem tip comparatively to the intact situation, the risk of end-of-stem pain might be minimized. SCOPE This pre-clinical study was accomplished using synthetic tibiae to experimentally predict the periosteal cortex strains at the proximal and stem tip regions, with a commercial press-fit stem and a new stem concept. CONCLUSIONS The results demonstrated that the new stem concept has the ability to minimize strain changes induced by the stem tip at the distal periosteal cortex and consequently, at the periosteal layer of bone tissue, which is highly pain sensitive, probably contributing to the reduction of the risk of end-of-stem pain.

Biomechanical analysis of total elbow replacement with unlinked iBP prosthesis: An in vitro and finite element analysis

BACKGROUND Numerous models of elbow prostheses are being used and can be divided into two categories: one being a semi-constrained, linked type; and the other being non-constrained, unlinked type. Recent reports of National Elbow Arthroplasty Registers reveal no significant differences in the survival rates between linked and unlinked prosthesis brands, and the main cause appointed for revision for both types is loosening. Some previous biomechanical studies confirm the presence of abnormal bone stresses for the linked type, which can be associated with the risk of loosening. However for the unlinked type, biomechanical studies are not available that corroborate a loosening risk. It seems, that issue has not yet been fully answered and requires further analysis. METHODS Cortex strains adjacent to the elbow joint were measured with strain gauges in synthetic humeri and ulnae, before and after replacement. To assess cancellous bone strains and cement stresses around the implant finite element models validated relative to measured strains were used. FINDINGS Bone strains adjacent to the implant tip increased several times in the humerus and ulna. At the epiphyseal regions a generalised cancellous bone strain reduction was observed for both humerus and ulna relatively to the intact bones. INTERPRETATION The unlinked elbow prostheses can be associated with the risk of bone fatigue failure by overload, particularly in the ulna, and bone resorption by stress-shielding at the epiphyseal regions. The identical structural behaviour relative to linked prostheses associated with the same loosening risks corroborates the results of recent arthroplasty published register reports.

Tibial periprosthetic fracture combined with tibial stem stress fracture from total knee arthroplasty

Total knee arthroplasty complications related to the prosthetic material are very rare, except for polyethylene wear. We report the case of a 58-year-old woman who came to the emergency service of our hospital with a periprosthetic tibial fracture (Mayo Clinic type I). Careful examination showed that this fracture was concomitantly associated with a tibial stem fatigue fracture. The prosthesis and the stem were sent to an independent biomechanics laboratory for evaluation. A finite-element CAD system was used to make a reconstruction, so as to ascertain whether there had been any manufacturing defect and what the causes of the event might have been. After evaluation of several hypotheses, it was concluded that the fracture in the prosthetic material had been caused by overloading at the plate/stem transition zone secondary to previous bone failure (fracture). From the evaluation of this case, the need to make appropriate assessment of bone mineralization can again be emphasized. In cases of doubt, a longer stem should be used.

Continuous zoom calibration by tracking salient points in endoscopic video.

Many image-based systems for aiding the surgeon during minimally invasive surgery require the endoscopic camera to be calibrated at all times. This article proposes a method for accomplishing this goal whenever the camera has optical zoom and the focal length changes during the procedure. Our solution for online calibration builds on recent developments in tracking salient points using differential image alignment, is well suited for continuous operation, and makes no assumptions about the camera motion or scene rigidity. Experimental validation using both a phantom model and in vivo data shows that the method enables accurate estimation of focal length when the zoom varies, avoiding the need to explicitly recalibrate during surgery. To the best of our knowledge this the first work proposing a practical solution for online zoom calibration in the operation room.

Maquet III procedure: what remains after initial complications - long-term results

BackgroundMaquet III procedure, unloved due to its complications (2% to 59%), has been progressively abandoned. At long-term follow-up, what happens to patients with complications that exceeded the initial ones (Acta Orthop Scand 60:20, 1989)? We retrospectively studied patients who were submitted to Maquet III procedure, by functional and radiologic long-term outcomes, in order to determine if this surgery has or has not fulfilled its initially proposed objectives. From 1970 to 1991, 116 patients benefit from the Maquet III procedure. From this, we were able to review in 2011, 23 patients (25 knees) who went through a single Maquet III procedure. Of these patients, 52% were males. Age at surgery was 39.7 ± 11.4, with a postoperative follow-up of 27.2 ± 3.1 years.MethodsA questionnaire has been prepared for collecting data, and it has been supplemented by clinical records. We evaluated the preoperative complaints, postoperative complications, and range of motion during the recovery time, as well as the postoperative pain-absence period. All patients underwent an objective assessment using the visual analog scale (VAS) at rest and activity, and the Kujala patellofemoral scoring system. A radiological assessment was also made in order to evaluate the arthrosis degree. The bicondylo-patellar angle described by Delgado-Martins (Arch Orthop Traumat Surg 96:303–304, 1980) was used to measure patellar tilt, and the Caton-Deschamps index to calculate the patellar height.ResultsOnly one knee had benefited from a total knee arthroplasty (20 years after the Maquet III procedure). Preoperative complains were mainly anterior knee pain, crepitus, and patellar instability. Nowadays, 10 patients (40%) still are pain free. Others had an average period without pain of 19.1 ± 6.1 years. VAS at rest was 1.7 ± 0.7 and in activity 4.4 ± 3.0. KPS was 61.9 ± 22.3 points. X-ray shows that 40% had a Kellgren-Lawrence grade of 1 at the patellofemoral joint.ConclusionMaquet proposed this technique for knee-pain relief, maintenance of the knee range of motion, and for slowly progressive osteoarthritic development. Viewed in a dispassionately way, we could notice that the initial objectives of this procedure were completely achieved. A part of 80% of the initial population was lost during follow-up, which may compromise the conclusions, perhaps, it is time to reflect again on this solution, so unloved by so many.