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The diagnostic value of the stump impingement reflex sign for determining anterior cruciate ligament stump impingement as a cause of knee locking
© Carmont et al.; licensee BioMed Central Ltd. 2012
Received: 16 September 2011
Accepted: 3 July 2012
Published: 28 August 2012
The stump impingement reflex is a subtle bounce to the knee thought to be caused by hamstring contraction when the knee is brought into extension and the torn ACL stump impinges between the distal femur and the tibial plateau. We have studied the diagnostic value of this sign.
From Feb 2008-Feb 2009, we audited 30 patients who underwent urgent arthroscopy for acutely locked knees. The presence of the stump impingement reflex prior to surgery was compared with the intra-operative findings. The diagnostic values of the stump impingement sign were found to be: Sensitivity 58%, Specificity 81%, Positive predictive value 70%, Negative predictive value 72% and Accuracy 71%.
We believe that the stump impingement reflex is a specific sign for ACL stump impingement as a cause of knee locking. We recommend close inspection for this sign when examining locked knees.
A knee is considered locked when it acutely loses full extension, during both active and passive movements and is held or “locked” in flexion. The causes of knee locking may be related to intra-articular pathology  i.e. a mechanical block (86%)  or due to an acute haemarthrosis and hamstring muscle spasm [3, 4].
Both Noyes and Dehaven comment that a ruptured anterior cruciate ligament (ACL) is the commonest cause of locking (72%) and with bucket handle meniscal tears (62%, 15%) and loose bodies/chondral injuries (20%, 6%) may frequently occur in their respective series [5, 6]. Partial ruptures to the ACL may result in locking, although this is uncommon [4, 7, 8]. Accurate clinical examination is essential as the presence of intra-articular impingement necessitates urgent arthroscopy.
The clinical examination of a painful sensitive locked knee can be difficult . Lachmans test and the pivot shift are very sensitive  although these findings are may be less accurate in the presence of an acute effusion  and full extension may be considered necessary to perform a pivot shift test .
Additional file 1: Video showing the subtle bounce of the knee caused by hamstring contraction in response to a stump of the ACL being stimulated by impingement within the nearside knee. (AVI 24 MB)
We aim to determine the diagnostic value of the stump impingement reflex sign in patients with acutely locked knees.
We undertook a prospective audit on all patients who presented with a locked knee following acute injury from February 2008 to February 2009. The hospital’s Audit Committee granted approval for the study. This studyhas received approval from the audit committee of the Robert Jones and Agnes Hunt Orthopaedic Hospital.
The arthroscopic findings were noted specifically looking for meniscal tears, loose bodies and ligament injury. The anterior cruciate ligament was assessed for the presence of injury and for stump impingment (Truth results). This allowed the diagnostic features of the test to be determined .
From Feb 2008-Feb 2009, 30 patients underwent urgent arthroscopy for acutely locked knees. The mean age was 25 years (16–56), 23 were male and 7 female, 16 patients had injured the left knee, 14 the right knee. At data collation 2 patients were noted to have previously undertaken ACL reconstruction and were excluded from data interpretation.
Pre-operatively 37% (10) patients demonstrated the stump impingement reflex (Figure 1), the remaining 20 did not. The mean fixed flexion deformity was 12 deg and flexion 120 deg.
Arthoscopic diagnoses of the causes of knee locking
ACL complete rupture
ACL partial rupture
Meniscal tear Lateral
Meniscal tear Medial
ACL & Medial Meniscus
ACL & Lateral Meniscus
No structural pathology/Haemarthrosis/Pseudolocking
Determination of the diagnostic validation of the stump impingement reflex sign
Scope (Truth) Positive
Scope (Truth) Negative
Exam (Test) Positive
Exam (Test) Negative
The diagnostic values of the stump impingement sign were found to be: Sensitivity 58%, Specificity 81%, Positive predictive value 70%, Negative predictive value 72% and Accuracy 71%.
Clinical examination of the acute knee aims to identify the diagnosis and also determine which knees are likely to benefit from urgent arthroscopy or focussed early physiotherapy. An acutely locked knee is uncomfortable, makes mobilisation difficult and physiotherapy painful. The recognition of the stump impingement reflex sign suggests a diagnosis of ACL impingement and so identifies those knees requiring prompt debridement. Stump debridement will allow the knee to settle and permit full extension improving outcome of ACL reconstruction  or permit physiotherapy rehabilitation. An acute haemarthrosis within the knee leads to the neurological inhibition of vastus medialis obliqus (VMO) resulting in weakness and atrophy [15, 16]. In addition full knee extension is required to perform VMO exercises, optimising neuromuscular facilitation and promoting recovery .
The Stump Impingement Reflex Sign is a subtle one, detected by close inspection and appreciated by clinical experience or specific instruction. The sign is an objective test raising the possibility of inter and intra-observer error. To accumulate of an adequate number of acutely locked knees in one clinic would require a large patient population, a large clinical practice and prompt referral and so would be difficult. Also to delay surgery in these patients for error determination would be unethical. Repeated examination of an individual’s painful knee may well lead to guarding of the knee with quadriceps contraction further reducing the diagnostic value further. The mobility of the ACL stump also influences the reliability of this sign. The stump may return to its anatomical position at any time between and during individual extension examinations thereby not impinging.
Consistent with other series [5, 6], the most common cause of knee locking was found to be a completely torn ACL (30%). Isolated lateral longitudinal bucket handle meniscal tears (23%) were more common than medial tears (10%) and isolated partially torn ACL injuries were least common (6.7%). Two patients had tears to the ACL in combination with bucket handle meniscal tears, one lateral meniscus, and one medial. Almost a quarter (23%) of patients had no evidence of meniscal injury and had either a normal ACL or a strained but intact ACL without impingement (a pseudo locked knee). Since Hilton’s initial work , the appreciation of the neurological innervation of individual intra-articular structures is only just being appreciated. Electromyographic observation of hamstring muscle activity following ACL stimulation has led to the recognition of an ACL-Hamstring reflex arc  whereas no reflex activity has been shown with meniscal stimulation . The fact that only 1 out of 28 patients demonstrated the reflex under general anaesthesia suggests that the knee flexion response is centrally controlled possibly via pain receptors within the ACL stump .
ACL grafts have been shown to reform a hamstring reflex arc between 37–80 months following bone-patellar tendon-bone autograft ACL reconstruction . Two patients had reinjured their knees at 42 and 96 months following reconstruction and if included, the specificity would increase to 83% and the negative predictive value to 75%.
We have determined the following diagnostic value of the stump impingement reflex sign: Sensitivity 58%, Specificity 81%, Positive predictive value 70%, Negative predictive value 72% and Accuracy 71%. We recommend close inspection for this sign when examining locked knees in the clinical setting. Identificaton of the reflex in the acutely painful locked knee may be of diagnostic value when other clinical signs may be less reliable.
All patients gave their consent for inclusion in the study and data collection.
We would like to thank Andrea Bailey and Jane Hughes, physiotherapists at the Sports Surgery Unit, the staff of the Day Surgery Unit and the Francis Costello Library at the Robert Jones and Agnes Hunt Hospital for their help with this research.
We have received no funding for this study.
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