But the Harmonized Standards are not mandatory!!!
O.K. let's start from the beginning:
There is no law, which can enforce anybody to apply any harmonized standards.
But during the assessment of conformity of your products "it helps" to follow/apply harmonized standards. Because if you declare the application of a harmonized standard, the notified body is allowed to apply the "assumption of equivalent safety".
If you choose to make your own procedure, the notified body has to evaluate, if your method is equivalent to the harmonized standard. Usually this takes time, and time is money.
And now for the Clinical Evaluation:
As a manufacturer of a medical device, you have to fulfill the requirements of Annex X of the MDD 93/42/EEC:
1. General provisions
1.1. As a general rule, confirmation of conformity with the requirements
concerning the characteristics and performances referred to in Sections 1
and 3 of Annex I under the normal conditions of use of the device and
the evaluation of the undesirable side-effects must be based on clinical
data in particular in the case of implantable devices and devices in Class
III. Taking account of any relevant harmonized standards, where appropriate,
the adequacy of the clinical data must be based on:
1.1.1. either a compilation of the relevant scientific literature currently available
on the intended purpose of the device and the techniques employed as
well as, if appropriate, a written report containing a critical evaluation
of this compilation;
1.1.2. or the results of all the clinical investigations made, including those
carried out in conformity with Section 2.
1.2. All the data must remain confidential, in accordance with the provisions
of Article 20.
If you want to save the money for the standard, you should at least follow
MEDDEV. 2.7.1 (attached). There can also be some requirements and procedures defined by the competent authority for clinical trials. Such requirements are usually specified in the national medical device law (and by-laws).
But: If you can show, that your product is clinical, technical and biological equivalent to a product, which is already on the market and you provide relevant literature (see clause 4.3. Literature route of MEDDEV 2.7.1) you do not need to perform a clinical trial, you can provide a compilation of the relevant scientific literature (as specified in Annex A of EN ISO 14155-1, that is almost the same text as in MEDDEV 2.7.1)