The Inconsistency of Experts and the Failure to Apply the Legal Tests

In a recent High Court case[1] concerning allegations of clinical negligence in relation to a hip replacement surgery, the judge concluded that the Defendant’s expert was ‘lacking credibility and understanding as to the applicability of the legal test’, and found the expert was not ‘an impressive witness’.  We must accept of course that often, once a judge decides to prefer the evidence of one expert over another, the justification may be in harsh terms.  But it is educative to consider what caused the judge to form this view in this particular case.

The case concerned a total hip replacement that failed and was identified as having failed the day after surgery when on x-ray dislocation was confirmed.  The agreed position of the experts in the joint statement  was that ‘we both agree that on the balance of probabilities the cause [of the post-operative dislocation] was a loose acetabular component’ and that ‘on the balance of probabilities an acceptable and competent press fit [of the acetabular component] was not secured intraoperatively’.  This agreement might have been thought to have resolved the issue on breach of duty;  if the operating surgeon had not achieved an ‘acceptable and competent’ degree of security to the component that might be thought to have been equivalent to a statement by both experts that the degree of security achieved fell below that reasonably to be expected of a reasonably competent surgeon in that field (the Bolam test).  Indeed that is how the Claimant’s counsel opened the case.  However it transpired that the Defendant’s expert did not mean to criticise the performance of the surgery, but merely intended thereby to describe the result, without comment on the standard of care demonstrated.

Answering the right question

The first lesson from this case is the importance of posing the right questions for liability experts to address in a joint statement and the importance of liability experts providing answers which address the relevant legal test.  Drafting the questions in the agenda for a joint discussion is the primary responsibility of the lawyers but the competent medico-legal expert must be aware of the legal tests which his or her evidence is to address, and in a clinical negligence claim that will (almost always) include addressing the Bolam test of negligence.  The question posed here was arguably ambiguous – ‘Was an acceptable and competent pressfit secured?’ Did that refer to performance by the surgeon or merely outcome?  Nowhere in the Bolam test does the word ‘acceptable’ or ‘competent’ appear.

Changing opinion

The second lesson is that the expert who changes his or her opinion is at risk of being found an unreliable expert – and that risk is greater the later the change occurs.  A change of opinion certainly needs rational justification.  In this case the change came in response to formal questions put to an expert by his own side after the joint statement and after a conference with counsel, and in the days before trial.  (These could not have been questions under CPR 35.6 which relates only to questions put to another party’s expert and imposes time limits on the asking of questions.)  In this case this process appears to have been used to put into the open answers which the expert had been prepared to give in conference to his own team on further discussion of, and clarification of, the joint statement and its implications.

In these answers the expert resiled from the previously agreed position that it was more likely than not that a loose acetabular cup had caused the subsequent dislocation, making this now only ‘a possibility’, moving, it appears, from a 51% likelihood to a 50:50 possibility.  He put forward 3 other explanations (soft tissue impingement, a haematoma and abnormal movement of the leg), regarding each of the explanations as equally possible.  The expert’s further suggestion that it was ‘very possible’ that a reasonable press fit had been obtained at surgery but that it was only loosened on dislocation of the hip the following day ‘as stated in my [earlier] report’ was not something that the judge was able readily to identify in that earlier report.

The judge noted that ‘the court should be slow to criticise an expert who does change an opinion, and prefers a frank expression of view rather than an unswerving adherence to an original position’.  But the judge found that the expert’s reasoning for his new position ‘disjointed’ and that he was ‘endeavouring to add plausibility to what were essentially very remote possibilities’ and that he had ‘strained to identify possibilities which in most respects are so remote as to be fanciful.’  Most damning, the judge concluded that the expert ‘lacked credibility and understanding as to the applicability of the legal test by resiling so dramatically and so recently from a position jointly adopted’,  while accepting that that these opinions were held ‘genuinely and honestly’.


The expert, as well as the instructing lawyer, must be aware of the legal test(s) applicable to the matters on which opinion is being sought and the expert must give opinion by reference to those legal tests, for example the balance of probabilities and the Bolam test.  And while it is only right that, in accordance with the expert’s duty to the court, any change of opinion in the course of the expert’s involvement should be communicated at the earliest opportunity, the time for careful consideration of the case, its facts, alternative explanations, the reasoning process and the opinion itself is when preparing a written report.  While an ‘unswerving adherence’ to an opinion no longer confidently held must be avoided, any subsequent changed opinion will need to be justified in a similar and convincing manner if the expert expects the changed opinion to carry weight in court.

[1] Pullen v Basildon & Thurrock University Hospitals NHS Foundation Trust [2015] EWHC 3134

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