MRI of the rotator interval of the shoulder

The rotator interval of the shoulder joint is located between the distal edges of the supraspinatus and subscapularis tendons and contains the insertions of the coracohumeral and superior glenohumeral ligaments. These structures form a complex pulley system that stabilizes the long head of the biceps tendon as it enters the bicipital groove of the humeral head. The rotator interval is the site of a variety of pathological processes including biceps tendon lesions, adhesive capsulitis and anterosuperior internal impingement. This article describes the anatomy, function and pathology of the rotator interval using magnetic resonance imaging (MRI).

MR Imaging Evaluation of the Bone Marrow and Marrow Infiltrative Disorders of the Lumbar Spine

The use of MR imaging in assessing lumbar bone marrow first requires an understanding of the bone marrow's normal composition and the various imaging sequences available for use. One of the most useful sequences is the T1-weighted spin-echo sequence. This sequence may be combined with other sequences such as T2-weighted or diffusion-weighted sequences; techniques such as fat suppression, chemical shift imaging, and contrast-enhanced imaging are discussed. The varying features of normal lumbar marrow related to the normal physiologic changes that occur with aging and with changes in hematopoietic demand are important to understand and are described. The appearances of infiltrative marrow disease are explained on the basis of marrow composition and whether disease causes proliferation, replacement, or depletion of normal marrow components.

Ultrasound guided dry needling and autologous blood injection for patellar tendinosis

OBJECTIVE

To evaluate the efficacy of ultrasound guided dry needling and autologous blood injection for the treatment of patellar tendinosis.

DESIGN

Prospective cohort study.

SETTING

Hospital/clinic based.

PATIENTS

47 knees in 44 patients (40 men, 7 women, mean age 34.5 years, age range 17 to 54 years) with refractory tendinosis underwent sonographic examination of the patellar tendon following referral with a clinical diagnosis of patellar tendinosis (mean symptom duration 12.9 months).

INTERVENTIONS

Ultrasound guided dry needling and injection of autologous blood into the site of patellar tendinosis was performed on two occasions four weeks apart.

MAIN OUTCOME MEASURES

Pre- and post-procedure Victorian Institute of Sport Assessment scores (VISA) were collected to assess patient response to treatment. Follow up ultrasound examination was done in 21 patients (22 knees).

RESULTS

Therapeutic intervention led to a significant improvement in VISA score: mean pre-procedure score = 39.8 (range 8 to 72) v mean post procedure score = 74.3 (range 29 to 100), p<0.001; mean follow up 14.8 months (range 6 to 22 months). Patients were able to return to their sporting interests. Follow up sonographic assessment showed a reduction in overall tendon thickness and in the size of the area of tendinosis (hypoechoic/anechoic areas within the proximal patellar tendon). A reduction was identified in interstitial tears within the tendon substance. Neovascularity did not reduce significantly or even increased.

CONCLUSIONS

Dry needling and autologous blood injection under ultrasound guidance shows promise as a treatment for patients with patellar tendinosis.

Sonography and MRI of Rectus Abdominis Muscle Strain in Elite Tennis Players

OBJECTIVE

The objective of this study was to describe the imaging findings at sonography and MRI of rectus abdominis muscle strain in tennis players.

CONCLUSION

Asymmetrical hypertrophy of the recti is seen in elite tennis players. The muscle belly hypertrophies on the side opposite the dominant arm and is subject to muscle tears of its deep fibers below the umbilicus. Imaging can be used to show these injuries.

When physiology becomes pathology: the role of magnetic resonance imaging in evaluating bone marrow oedema in the humerus in elite tennis players with an upper limb pain syndrome

Upper limb stresses are well recognised in tennis, and the normal physiological responses of the humerus to externally applied forces are well defined. Changes to both the microscopic and macroscopic bony architecture are often not apparent on plain radiographs in the early stages of a stress reaction. Bone scintigraphy is more sensitive, but not very specific to subtle changes, as is computer aided tomography. Magnetic resonance imaging (MRI) scans are now used to assess many musculoskeletal injuries, and may allow earlier recognition of changes. This study reports on eight high level tennis players (seven professionals, one highly ranked amateur), who all presented during the course of the 2002 Australian Open and its lead up events. All had an upper limb pain syndrome centred around the elbow. Each underwent an MRI scan. A group of asymptomatic players also had scans of the same area to serve as controls. All symptomatic patients had various levels of bone marrow oedema in their distal humerus, some with periosteal reactions, although most without. No scans revealed a cortical breach. The management of these reactions is problematic in players on an international circuit, as review is sporadic and not always coordinated. The timetabling of international tournaments means it is difficult for top players to obtain adequate rest to avoid injury. Awareness of this early overuse reaction needs to be raised to allow preventive and therapeutic options to be considered to reduce the incidence of this humeral stress response producing an upper limb pain syndrome.

Imaging of the posterolateral corner of the knee

The structures of the posterolateral corner of the knee are increasingly recognized as fulfilling an important role in maintaining knee stability. The posterolateral aspect of the knee is stabilized by a complex anatomy of osseous, myotendinous and ligamentous structures. Unrecognized injuries to this part of the knee are a cause of failure of cruciate ligament reconstruction. This review focuses on the anatomy and common injury patterns involving the posterolateral corner of the knee, with emphasis on magnetic resonance imaging.

MR imaging findings of acetabular dysplasia in adults

OBJECTIVE

To evaluate the diagnostic accuracy of MR imaging in the identification of labral and articular cartilage lesions in patients with acetabular dysplasia.

DESIGN AND PATIENTS

Pre-operative MR imaging was performed on 27 hips in 25 consecutive patients (16 males, 9 females, age range 19-52 years, mean age 31.2 years) with radiographic evidence of acetabular dysplasia (centre-edge angle of Wiberg <20 degrees). The average duration of symptoms was 16.2 months. Two musculoskeletal radiologists assessed MR images in consensus for the presence of abnormality involving the acetabular labrum and adjacent acetabular articular cartilage. A high resolution, non-arthrographic technique was used to assess the labrum and labral chondral transitional zone. Surgical correlation was obtained in all cases by a single surgeon experienced in hip arthroscopy and ten patients with normal hip MRI were included to provide a control group.

RESULTS

The acetabular labra in the dysplastic hips demonstrated abnormal signal intensity, and had an elongated appearance when compared with the control group (mean length 10.9 mm vs 6.4 mm). Morphological appearances in the labra included surface irregularity, fissures and cleft formation. MR imaging correctly identified the severity of chondral abnormality in 24 of 27 hips (89%) when compared with arthroscopic findings.

CONCLUSIONS

MR imaging demonstrates an elongated labrum, focal intra-substance signal change and irregularity and fissuring of the margins in patients with acetabular dysplasia. Abnormality is also identified at the labral chondral transitional zone, where fissuring, focal clefts, chondral deficiency and subchondral cyst formation may be apparent. A high-resolution, non-arthrographic technique can provide an accurate preoperative assessment and evaluate the presence of premature osteoarthritis.

Imaging of hamstring injuries: therapeutic implications

Though recent research into the diagnosis and management of hamstring disorders has resulted in early and accurate recognition of injury, hamstring strain remains the most common form of muscle injury in the active population. With prompt recognition of hamstring strain, an appropriate rest and rehabilitation routine may be devised by the sports clinician in the hope of avoiding future and possibly more debilitating injury. As such, imaging has played a pivotal role in assisting athletes, both elite and recreational, in returning to activity expeditiously.

Overview of the National Health Educator Competencies Update Project, 1998-2004

The National Health Educator Competencies Update Project (CUP), conducted during 1998-2004, addressed what health educators currently do in practice, the degree to which the role definition of the entry-level health educator is still up-to-date, and the validation of advanced-level competencies. A 19-page questionnaire was sent to a representative sample of health educators in recognized practice settings in all states and the District of Columbia. A total of 4,030 health educators participated in the research (70.6% adjusted response rate) resulting in the largest national data set of its kind, with 1.6 million data points. The model derived from the research was hierarchical (7 areas of responsibility, 35 competencies, and 163 subcompetencies), with three levels of practice (Entry, Advanced 1, and Advanced 2) differentiated by degrees earned and years of experience. The findings affect professional preparation, credentialing, and professional development.

Adhesive capsulitis: sonographic changes in the rotator cuff interval with arthroscopic correlation

OBJECTIVE

To evaluate the sonographic findings of the rotator interval in patients with clinical evidence of adhesive capsulitis immediately prior to arthroscopy.

DESIGN AND PATIENTS

We prospectively compared 30 patients with clinically diagnosed adhesive capsulitis (20 females, 10 males, mean age 50 years) with a control population of 10 normal volunteers and 100 patients with a clinical suspicion of rotator cuff tears. Grey-scale and colour Doppler sonography of the rotator interval were used.

RESULTS

Twenty-six patients (87%) demonstrated hypoechoic echotexture and increased vascularity within the rotator interval, all of whom had had symptoms for less than 1 year. Three patients had hypoechoic echotexture but no increase in vascularity, and one patient had a normal sonographic appearance. All patients were shown to have fibrovascular inflammatory soft-tissue changes in the rotator interval at arthroscopy commensurate with adhesive capsulitis. None of the volunteers or the patients with a clinical diagnosis of rotator cuff tear showed such changes.

CONCLUSIONS

Sonography can provide an early accurate diagnosis of adhesive capsulitis by assessing the rotator interval for hypoechoic vascular soft tissue.

Hamstring Muscle Complex: An Imaging Review

Increasing activity in the general population and the high demands placed on athletes have resulted in injuries to the hamstring muscle complex (HMC) being commonplace in sports. Imaging of HMC injuries can form a considerable part of a sports medicine practice, with a wide spectrum of such injuries being reflected in their varied imaging appearances. Magnetic resonance (MR) imaging and ultrasonography (US) are the imaging modalities of choice in this setting. Both MR imaging and US provide exquisitely detailed information about the HMC with respect to localization and characterization of injury. Optimization of MR imaging involves the use of a surface coil and high-resolution techniques, allowing the musculoskeletal radiologist not only to diagnose injury and assess severity but also to provide the clinician with useful clues with respect to prognosis. The portability and availability of US make it an attractive modality for the diagnosis of acute hamstring injuries, although its effectiveness is dependent on operator experience. A thorough knowledge of the HMC anatomy and of the spectrum of imaging findings in HMC injury is crucial for providing optimal patient care and will enable the musculoskeletal radiologist to make an accurate and useful contribution to the treatment of athletes at all levels of participation.

Magnetic resonance imaging of the hip: detection of labral and chondral abnormalities using noncontrast imaging

PURPOSE

Traditional imaging techniques have limited ability to detect subtle chondral and labral injuries of the hip. We performed a retrospective review of patients who underwent magnetic resonance imaging (MRI) of the hip and subsequent hip arthroscopy in order to evaluate the ability of optimized, noncontrast MRI to identify tears of the acetabular labrum and defects in articular cartilage.

TYPE OF STUDY

Retrospective review of a consecutive sample.

METHODS

Between January 1997 and July 2000, 92 patients had MRI of the hip, followed by arthroscopic surgery of that hip by 1 of 2 surgeons (R.B., D.E.P.). Two musculoskeletal MR radiologists blinded to the initial MRI and surgical findings, independently interpreted the studies, looking for the location and degree of articular cartilage and acetabular labral pathology.

RESULTS

Of the 92 patients studied, each of 2 radiologists correctly identified 83 (94%) and 84 (95%) of the 88 labral tears present at surgery, respectively. There was 92% interobserver agreement on the MRI studies. For articular cartilage defects on the femoral head and acetabulum, there was good agreement (92% and 86% within 1 grade) between MRI and surgical grading and between the 2 MR readers (kappa of 0.8 for femoral head cartilage and 0.7 for acetabular cartilage).

CONCLUSIONS

This study shows that noncontrast MRI of the hip, using an optimized protocol, can noninvasively identify labral and chondral pathology. Such information may facilitate deciding which patients warrant surgical intervention, thus preserving hip arthroscopy as a therapeutic tool.

LEVEL OF EVIDENCE

Level II, Development of Diagnostic Criteria Study.

A comparison of magnetic resonance imaging, sonography, and radiography of the hand in patients with early rheumatoid arthritis

OBJECTIVE

As therapy for rheumatoid arthritis (RA) becomes more effective, more sensitive imaging methods are required to assess disease activity and joint damage. We compared magnetic resonance imaging (MRI), sonography, and radiography for assessment of disease activity for the detection of bony erosions.

METHODS

Forty-six patients with newly diagnosed RA (onset within 2 years) received clinical and laboratory assessment followed by radiographs, sonography, and MRI of the right hand at baseline and at 6 months according to a standardized protocol. We determined the presence of edema, synovitis, effusions, tendon fluid, tendon thickening, and size in the same way by MRI and sonography. The intra- and interreader reliability of MRI and radiographs and predictors of MRI erosions at 6 month followup were also examined.

RESULTS

At baseline, 39 (85%), 14 (30%), and 17 (37%) patients had erosions identified on MRI, sonography, and radiography, respectively. Over time, the percentage of patients with erosions increased to 91% for MRI, 41% for sonography, and 48% for radiography. The absolute number of erosions increased from 177 to 239 erosions for MRI, from 30 to 43 for sonography, and from 38 to 73 for radiographs. The intra- and interreader reliability for the assessment of erosions and synovitis on MRI was acceptable (intrareader ICC of 0.60 and 0.90; interreader ICC of 0.77 and 0.89, respectively).

CONCLUSION

MRI appears to be the most sensitive modality for erosive disease compared with sonography and radiography. Sonography detected more joint and tendon sheath effusions than MRI in this study and therefore may have a role in the assessment of disease activity.

Evaluation of the immortalised mouse brain capillary endothelial cell line, b.End3, as an in vitro blood–brain barrier model for drug uptake and transport studies

Well-characterised cell lines represent important tools for the study of endogenous solute or xenobiotic transport. A brain microvascular cell line, b.End3, isolated from mice transformed with the Polyoma virus middle T-antigen is available commercially. Here we report the characterisation of some features of b.End3 of relevance to its use in blood-brain barrier transport investigations. The b.End3 cells displayed a distinctive spindle-like squamous morphology in culture. Clathrin coated pits and numerous uncoated intracellular vesicles were evident within the cells, as was the expression of the vesicle-associated proteins, clathrin, caveolin-1, flotillin and dynamin II. In the presence of C6 astroglial co-culture b.End3 monolayers achieved a maximal transendothelial electrical resistance of 130 Omega cm2, but lacked real discrimination with respect to the permeation of transcellular and paracellular probes, e.g. permeability coefficients (x 10(-6) cm s(-1)) for propranolol of approximately 23 vs. 16 for sucrose. RT-PCR analysis confirmed the presence within the b.End3 cells of mRNA transcripts for the following transporters: GLUT-1; MCT 1 and 2; OAT1; Oatp1; mdr 1a and 1b; MRP 1 and 5; beta-alanine, system L and system y+L amino acid carriers; the nucleoside transporters cNT1 and 2, eNT1 and 2, and the tight junctional elements, ZO-1, JAM, occludin, claudin-1 and -5. The b.End3 cells actively accumulated D-glucose in a sodium-independent manner with characteristics consistant with that of GLUT-1. Functionality for P-glycoprotein efflux was evident as assessed by a rhodamine-123 accumulation and retention assay. The system L LAT1/4F2hc amino acid transporter was examined through uptake of L-leucine and L-phenylalanine and provided Km and Vmax values of approximately 16 microM and 350-480 pmol/mg protein/10 min, respectively; the affinity of transport for these substrates being weaker, approximately threefold, when the b.End3 cells were grown in the presence of C6 astroglial factors. Although the b.End3 cells appear unsuitable for transendothelial permeability assessments they display characteristics that would allow their worthwhile use in studies addressing blood-brain barrier transport mechanisms.

Evaluation of the hamstring muscle complex following acute injury

OBJECTIVE

To evaluate the imaging findings following acute hamstring injury.

DESIGN AND PATIENTS

We retrospectively reviewed the imaging findings of hamstring muscle complex (HMC) strain in 170 patients referred to our institution over a 3-year period. A total of 179 injuries to the HMC were demonstrated in 170 patients (154 male, 16 female, mean age 28.2 years). The mean duration of symptoms was 4.7 days (range 1-10 days). MR imaging was performed in 97 cases and sonography in 102 cases (both modalities were performed in 20 examinations). Attention was directed to the frequency of muscle involvement, the location of the injury within the muscle-tendon unit, the extent of the injury and discriminating avulsion from muscle injury.

RESULTS AND CONCLUSIONS

Twenty-one patients had proximal tendon injury, with sixteen avulsions and five partial tears. Sixteen of these patients had surgical confirmation of hamstring avulsion from the ischial tuberosity (14 conjoint, 2 biceps femoris alone) and all were reliably diagnosed with MR imaging (16/16), but less so with sonography (7/12). Four distal tendon avulsions were also observed (three semitendinosus, one biceps femoris). With respect to muscle injury, the biceps femoris was most commonly injured (124/154). Semimembranosus was an uncommon muscle injury (21/154) and semitendinosus rare (9/154). Imaging can discriminate a hamstring tendon avulsion from musculotendinous strain and helps identify which patients necessitate surgical management as opposed to conservative treatment.

Prospective clinical study of autologous chondrocyte implantation and correlation with MRI at three and 12 months

In order to determine the usefulness of MRI in assessing autologous chondrocyte implantation (ACI) the first 57 patients (81 chondral lesions) with a 12-month review were evaluated clinically and with specialised MRI at three and 12 months.

Improvement 12 months after operation was found subjectively (37.6 to 51.9) and in knee function levels (from 85% International Cartilage Repair Society (ICRS) III/IV to 61% I/II). The International Knee Documentation Committee (IKDC) scores showed an initial deterioration at three months (56% IKDC A/B) but marked improvement at 12 months (88% A/B).

The MRI at three months showed 82% of patients with at least 50% defect fill, 59% with a normal or nearly normal signal at repair sites, 71% with a mild or no effusion and 80% with a mild or no underlying bone-marrow oedema. These improved at 12 months to 93%, 93%, 94% and 91%, respectively.

The overall MR score at 12 months suggested production of normal or nearly normal cartilage in 82%, corresponding to a subjective improvement in 81% of patients and 88% IKDC A/B scores. Second-look surgery and biopsies in 15 patients (22 lesions) showed a moderate correlation of MRI with visual scoring; 70% of biopsies showed hyaline and hyaline-like cartilage. Thus, MRI at 12 months is a reasonable non-invasive means of assessment of ACI.

Posterior tibiotalar ligament injury resulting in posteromedial impingement

OBJECTIVE

To describe the imaging features of the normal anatomy and injury of the posterior tibiotalar ligament (PTTL) of the ankle in a group of patients presenting with posteromedial impingement.

MATERIALS AND METHODS

Twenty-five consecutive patients underwent imaging for posteromedial ankle pain following injury. All 25 patients were investigated with MR imaging, 17 with ultrasound and 5 with radionuclide bone scanning. Clinical data and surgical findings were correlated with the results of radiological investigation.

RESULTS

The PTTL demonstrated scar formation and a hypertrophic response consistent with previous injury in all 25 patients. Displacement or encasement of the surrounding tendons and their sheaths was often encountered (12/25). Bony avulsion was demonstrated in 5/25 occasions with ultrasound and 2/25 with MR imaging. Twelve patients underwent surgery, confirming the radiological findings.

CONCLUSION

Imaging can effectively evaluate the integrity of the PTTL in the context of ongoing posteromedial ankle pain.

Adhesive capsulitis: role of MR imaging in differential diagnosis

The purpose of this study was to describe and characterize the MR imaging findings in a group of patients who underwent surgery for adhesive capsulitis. Twenty-four MR imaging studies in 24 consecutive patients with clinical evidence of adhesive capsulitis were performed prior to arthroscopic capsulotomy. There were 17 women and 7 men with a mean age of 53.5 years. Images were scrutinised for changes in the synovium particularly in the rotator interval, around the biceps anchor and axillary pouch. Intravenous gadolinium was given routinely. We also examined a control group of 22 patients who underwent the same MR imaging protocol after referral for rotator cuff pathology. Soft tissue density showing variable enhancement after gadolinium administration was visible in the rotator interval in 22 of 24 studies on MR imaging. Seventeen patients showed soft tissue density partially encasing the biceps anchor. Ten patients showed thickening and gadolinium enhancement of the axillary pouch. Three patients from the study cohort had partial tears of the supraspinatus tendon. All the patients subsequently had surgery which confirmed fibrovascular scar tissue in the rotator interval, around the biceps anchor and a variable degree of synovial inflammation of the glenohumeral capsule. Two patients from a control group with suspected rotator cuff pathology showed abnormal intensity in the rotator interval on MR imaging. Magnetic resonance imaging can identify changes in the shoulder joint that correspond to abnormalities seen at surgery. This may be useful for discriminating adhesive capsulitis from other causes of shoulder pain.

Magnetic resonance imaging of the wrist ligaments

The advent of small arthroscopes has enabled the hand surgeon to visualize the ligaments of the wrist directly with resultant increased accuracy in diagnosing and treating pathology. Orthopaedists are now demanding a preoperative assessment and this, in turn, has necessitated that radiologists have a comprehensive understanding of wrist anatomy. High resolution MR imaging can identify the wrist ligaments reliably and provide information concerning their integrity. Interpretation is not straightforward; there is considerable anatomic variation, and there are perforations, defects and degenerative tears that can be troublesome in diagnosing injury. However, with experience and attention to anatomic detail, the radiologist can provide useful information regarding structural abnormalities. When injured, the ligaments of the wrist behave as other joint ligaments do. Findings following injury include discontinuity of normal striated bands, incomplete disruption, irregularities and alteration in normal signal. Fluid pooling around a ligament and concomitant bone injury are other clues to injury. The identification of such structural abnormalities may help to explain altered biomechanics and improve the management of patients following wrist injury.

Sonographic examination of lateral epicondylitis

OBJECTIVE

The purpose of this study was to describe the sonographic appearance of the common extensor origin in cadavers and asymptomatic volunteers, and to relate this appearance to the findings in patients with lateral epicondylitis.

SUBJECTS AND METHODS

Seventy-two elbows in 71 patients with lateral epicondylitis were examined on sonography. Most of the patients (60/71) gave a history of repetitive microtrauma. The injuries were evaluated with respect to location and severity. Focal areas of degeneration, discrete cleavage tears, and involvement of the lateral collateral ligament were identified. Calcification and bony changes were noted. The appearance of the normal common extensor tendon was described, and cadaveric specimens were dissected. Twenty-one patients subsequently underwent surgery.

RESULTS

The normal common extensor origin is composed of longitudinal fibrils bound closely with the extensor carpi radialis brevis constituting most of the deep fibers, with the extensor digitorum making up the superficial part. The lateral collateral ligament can be identified as a discrete and separate band. The most common appearance of lateral epicondylitis is a focal hypoechoic area in the deep part of the tendon (46/72). These focal areas were identified at surgery and corresponded histologically to collagen degeneration with fibroblastic proliferation. Often discrete cleavage planes traversing the tendon were manifest as partial (18/72) and complete (2/72) tears. The lateral collateral ligament was involved in eight of 72 elbows.

CONCLUSION

Sonography of the common extensor origin can be used to confirm lateral epicondylitis in patients with lateral elbow pain and provide information about the severity of the disease.

An intervention to increase fruit and vegetable consumption using audio communications: in-store public service announcements and audiotapes

Consumers make an estimated 70% of their food purchase decisions as they shop. Effective presentation of information about healthier food selections at the point-of-purchase should have an impact on their decisions. This study was designed to evaluate the effectiveness of two audio formats on knowledge, attitudes, and beliefs about fruits and vegetables and on intake of these foods. Participants identified as "regular shoppers" (n = 374) in three intervention stores were recruited as they entered. They provided baseline demographic data, answered questions about their knowledge and beliefs about fruits and vegetables, and completed a checklist to assess fruit and vegetable intake. They were given two one-hour audiotapes and asked to play them within the next four weeks. In-store public service announcements (PSAs) with information about fruits and vegetables were rotated every 30 minutes for four weeks. A control group (n = 378), recruited in three other stores matched by demographic characteristics, provided the same information. They received an audiotape with stress reduction information. At posttest, the original series of questionnaires were readministered in telephone interviews with 87.7% of the original intervention group and 93.7% of the original control group. Knowledge scores in the intervention group increased significantly over baseline and as compared with the control group. Self-reported fruit and vegetable intake increased significantly in both groups, perhaps in part because of a seasonal effect. The increase from baseline was significantly higher in the intervention group and compared with controls. These findings support the further exploration of the use of audiotapes in nutrition education interventions.

A model for non-specific toxicity with aquatic organisms over relatively long periods of exposure time

Experimental data have shown that the internal lethal concentrations of halobenzenes for aquatic organisms decreased with exposure time. In this paper, a model based on the concept of life expectancy reduction was developed to describe this relationship. The model was verified with experimental data for fish (Gambusia affinis) and juvenile crab (Porturius pelagicus(L)). It is proposed that long term non-specific toxicity can be measured as the reduction of the life expectancy of the exposed organism per unit internal concentration (or volume fraction) of the toxic compound. The model can be used to estimate internal lethal concentration at any given exposure period and vice versa. The model can also be used to estimate chronic values of the internal concentration, of the toxicants. It provides a useful tool for assessment of environmental risk of organic compounds in aquatic ecosystems.