Patents: cross-examination of expert witness

By Audrey Horton


The Court of Appeal has considered the effect of the decision not to cross-examine the patent owner's expert witness on his opinion on obviousness.


Generally, a party is required to challenge in cross-examination the evidence of any witness of the opposing party if he wishes to submit to the court that the evidence should not be accepted on that point. This rule gives the witness the opportunity of explaining any contradiction or alleged problem with his evidence. 

If a party has decided not to cross-examine on a particular important point, he will have difficulty submitting that the evidence should be rejected. However, the rule is not an inflexible one. Failure to put a relevant matter to a witness may be most appropriately remedied by the court permitting the recall of that witness to have the matter put to him (Phipson on Evidence, 19th Edition, 2016).


B sued E for infringement of two patents for artificial heart valves. 

The High Court held that one patent was valid and infringed, but that the other patent was invalid for obviousness.

B and E both called two expert witnesses: an interventional cardiologist, a clinician who might implant a replacement heart valve; and a bio-medical engineer, who might design one. There were three rounds of exchange of expert reports before the trial, although B's rejoinder report was made only by their bio-medical engineering expert.

E did not cross-examine B's cardiologist expert on the prior art. Both parties appealed.


The court dismissed the appeals. It held that although the rule on cross-examination set out in Phipson was important, it was not inflexible.

Where it was proposed to save time by not cross-examining two witnesses in relation to the same or similar subject matter, it was good practice for the matter to be raised with the court beforehand, so that it could give directions in the light of the parties' submissions. Directions should ensure fairness to the parties without incurring unnecessary costs by extending the length of the trial. However, the fact that such a direction was not sought or given did not automatically require the court to accept an unchallenged reason given by one expert.

The rule requiring important positive evidence to be challenged was, generally, a rule not simply for the benefit of the witness whose honesty or professional reliability was challenged, but was also designed to ensure the overall fairness of the proceedings for the parties. The rule applied with particular force where a witness gave direct evidence of a fact of which he had knowledge and which it was proposed to invite the court to disbelieve. Fairness to the witness and to the parties demanded that the witness should be challenged on his factual evidence so as to give him the opportunity of affirming or commenting on the challenge, or on a positive matter which it was proposed to set against his evidence.

Not every situation however called for a rigid application of the rule. At least part of the unfairness which the rule was intended to address was the lack of any opportunity for a witness to respond to a challenge to his evidence. Here B put in three rounds of expert evidence, so each expert had more than ample opportunity to comment on the views of the other. The potential for unfairness to the witness in these circumstances was much reduced.

In particular, B's expert evidence could not be regarded as unchallenged, because, by the time he went into the witness box, B's expert had seen and had the ability to comment on the contrary technical opinion of E's expert and had seen the same challenges put to B's other expert. This was a wholly different situation to one where a witness had no notice of the nature of the challenge to his evidence and no fair opportunity to comment on it. 

Although B's experts had different expertise, they were both giving evidence on how a collaborative skilled team would have undertaken a hypothetical design project, and were qualified to do so. Consistently with that, B's expert had expressed himself in his first report in terms which showed he considered he was able to speak on behalf of the whole team. So, there was a genuine overlap between the evidence of the two experts and both were making exactly the same technical points. The court was justified in thinking that B would not be able to identify any point which was exclusively for the clinician as each expert had claimed the ability to make the same points. Even if the overlap had been raised by E, the court should not have required both experts to be cross-examined on these points as this would have increased the length of the trial unnecessarily.


This decision shows how general rules of procedure and evidence may not always apply in the specialised circumstances of patent litigation. Here, as in most patent cases, there had been several exchanges of expert evidence. . Those advising the parties may have good reason for deciding to limit cross-examination of their opponent's expert witnesses. In these cases, it is advisable to raise this decision in advance so that both parties can raise concerns and the court can give directions in light of the submissions. Failure to seek directions re not cross-examining two witnesses on the same subject-matter did not mean the unchallenged evidence of one expert witness must be accepted.  Here, acceptance of the evidence of an expert witness who was not individually challenged in cross-examination did not render judgment unsafe.

Case: Edwards Lifesciences LLC & Ors v Boston Scientific Scimed Inc [2018] EWCA Civ 673.

First published in the May 2018  issue of PLC Magazine and reproduced with the kind permission of the publishers.  Subscription enquiries 020 7202 1200.